In Kahala, the S.W.A.T. team unloads while Steve tells them their target is Wo Fat and is a known terrorist. They have assessed the situation using thermal imaging and have determined there is only one person in the house. He walks back to the car and finds, CIA agent Jenna Kaye putting on her vest, but Steve handcuffs her to the squad car stating she isn't a field agent. 5-0 and S.W.A.T. approach the house, but when one of the S.W.A.T men breaks open the door, it sets off a trigger and a bomb goes off. Jenna hears the explosion and uses a nightstick in the squad car to free herself. Steve goes into the house and Kono tells him through the radio that Wo Fat is upstairs and he goes up the stairs. He goes into a bedroom and comes face to face Sang Min. Sang Min is surprised to see him and bolts off the balcony with Steve right behind him. Steve tells Kono through radio their new target is Sang Min as Sang Min runs away, shooting at him over his shoulder. Sang Min runs out into the street when he gets hit by the squad car Jenna's driving. Danny yells at her to get down as Sang Min shoots at the car and then at Danny, who returns fire. Sang Min runs away and Danny makes sure Jenna's okay, telling her to stay there. He chases after Sang Min who runs into another house and exits out the other side. Danny doesn't see him come out, so he goes into the house looking for Sang Min. Instead he finds a body on the kitchen floor. He leans down to check for a pulse.
Steve has approached the house when Sang Min busts through the garage door, driving the owners car. Steve shoots at the car, but Sang Min gets away. Steve gets the plate number, CT832R and puts it out over the radio. Chin and Kono join him and Steve tells them what kind of car it was and that if Sang Min knows they're onto him he's going to try and flee the island. Jenna joins them and Steve looks at the handcuff still on her wrist and asks, "Correct me if I'm wrong, but wasn't there a car attached to you?" She asks who Sang Min is and Chin tells her about him and Jenna suggests Sang Min was holing up at Wo Fat's safe house. Steve counters that Sang Min could've been taking a shot at Wo Fat, since the front door was rigged with an explosive that Sang Min was expecting Wo Fat. Sang Min is a loose end since escaping from prison and Wo Fat doesn't like loose ends.
They are interrupted by Danny who has come out of the house and is coughing heavily. He collapses as Steve runs over and asks what's wrong. Danny tells him he can't breathe and Steve takes his vest off and Jenna adds Danny's pulse is slow. Steve asks what happened and Danny struggles to tell him there's a body inside with no pulse. Jenna runs into the house and covers her mouth. Steve joins her but she stops him from going any further as whatever the man died from could be biological and they have to get Danny to a hospital right away. Kono calls to them from outside and when they get back out they see Danny is starting to convulse. Steve struggles to keep Danny with them.
Danny is being rushed into the hospital on a gurney as Steve is following close behind. They ask if they know what kind of toxin they're dealing with and Steve tells them his team is still working on it. They tell him to hurry because Danny doesn't have a lot of time.
Inside the house a HazMat team is inspecting the body and the spilled carton of milk. Outside, Jenna tells Chin and Kono the victim died from a chemical nerve agent, which she explains that she knows about from working with a CIA threat assessment team. She thinks it is a toxin called sarin and it was in the milk the victim drank and Danny was exposed when he touched him to feel for a pulse. Jenna calls Steve and tells him that it's sarin and if they don't get him on the right medications, he will die.
Chin, Kono and Jenna join Steve in the hospital. Steve tells them it's still early but Danny's responding to the drugs and it looks like he'll pull through. Steve tells Jenna she was right and if she hadn't told him when she did Danny would probably be dead. Jenna asks if Danny has a family and Steve tells her about Rachel and Grace and Chin adds Danny was supposed to have Grace this weekend as Rachel and Stan are out of town. Kono asks if they can see Danny and Steve tells her they can't yet and they need to I.D. the victim and find out where he bought the milk. He adds it's either an act of terrorism or a random act of mayhem, like the Tylenol scare in the 80s. They need to contact the milk distributors and have them pull all milk off the shelves. Kono and Chin leave and Steve tells Jenna to call the CDC, send them a sample and make sure nothing else is contaminated. He thanks her for saving Danny's life and she asks where he's going. He tells her he has a little something to take care of.
Or rather, a little someone as Steve pulls up in front of Grace's school and gets out of his car. While he's waiting he gets a call from Sang Min who muses that Steve was surprised to see him. He adds he was surprised to see Steve and Steve guesses the bomb was for Wo Fat. Sang Min answers they both missed their chance but asks what its worth for him to help him find Wo Fat. Steve answers it's not worth much as he already screwed them over once. Sang Min pushes for a deal and Steve hangs up on him. Grace comes outside and Steve waves to her. Grace greets him, "Uncle Steve" and gives him a hug. She asks where Danny is and Steve tells her he's going to take her to see him in the hospital. Grace asks if he's going to be okay and Steve tells her that her father is one tough and brave guy. He asks her if she can be brave like Danno and she nods yes.
At headquarters, Jenna tells Kono the milk is sold in over a hundred locations on the island. She continues stating that they have pulled the milk off the shelves and CDC is randomly testing them , but none are testing positive for sarin. Kono guesses this is an isolated incident but adds sarin is an odd choice for a murder weapon. Jenna tells her it's not as farfetched as one might think and it used to be really hard to get but since 9/11 they're taken down so many terrorists sarin brokers are looking for new clients, basically anyone with cash and a connection. Chin has come back with an I.D. on the victim, Amoka Mulitalo, who was arrested six months ago for trespassing . The house they found him in wasn't his own, but belongs to a Jeff and Sheila Fallon, who pay taxes in California and this house is their vacation home.
Chin goes and talks to the victim's daughter and she tells him after her father's auto accident, he had impaired memory and couldn't function properly so he moved in with her. Chin clarifies that she filed a missing person report for her father a few weeks ago and it wasn't the first time. She tells him sometimes her father would go for a walk and forget how to get home. If she couldn't find him she'd file a missing person report and then the police would find him on the street or in a shelter, except for this time.
Kono and Jenna Skype with the Fallons from headquarters but they don't recognize the victim or know why he'd be in their house. Kono tells them he was squatting and while there he drank some milk that contained sarin and was killed in minutes. The Fallons exchange a look and Sheila Fallon tells them they were supposed to come to Hawaii last week. She asked their caretaker Gabriel to stock the refrigerator, but they ended up canceling the trip. Jenna clarifies Gabriel is the one who bought the milk and Kono asks when the last time they spoke to him was. Jeff Fallon tells them it was a few days ago when they fired him for using their checkbook to write himself checks. He told Gabriel to keep the money and leave his key. Gabriel didn't take that well.
Jenna calls Steve and tells him the new development and that Gabriel the former caretaker has served time for aggravated assault and several home invasion robberies. Steve asks how a caretaker would get his hands on sarin and Jenna tells him Chin and Kono are on the way to ask Gabriel that. Steve is still in the hospital and Grace is sitting on a bench drawing a picture. Steve sees a doctor approaching and tells Jenna he has to go. The doctor tells Steve that Danny is ready for some visitors.
Danny looks like he is sleeping when Steve and Grace enter the room. Grace gives Danny a hug and shows him the picture she drew. Danny loves it and shows it to Steve and it's a picture of Danny and Grace. Steve asks how he's feeling and Danny tells him he feels like everything hurts and his head is about to explode like the worst hangover he's ever had. Grace innocently asks what a hangover is and Danny tells her it's something she'll learn about when she's older and Steve adds like thirty, or thirty-five or sixty. Danny thanks Steve and Steve tells him now that he's awake he'll go back to the office. He states that he'll pick Grace up later and she can stay with him until Rachel gets back. He asks if Danny's okay and Danny tells him he has everything he needs right there, looking at Grace. Steve leaves the room and "Party in the U.S.A." starts blasting from Grace's backpack. Danny asks if she kidnapped Miley Cyrus or if she got a cell phone. Grace tells him that Stan gave it to her and Danny isn't happy with this latest gift and tells Grace the first boy who calls her he's throwing it in the ocean. Grace answers her phone and it's Rachel. Danny tries to signal her not to tell, but Grace tells her that Danny's in the hospital. She tells Danny that mommy wants to talk to him and hands the phone over. Danny talks to Rachel and insists that he's fine.
Kono and Chin enter a diner and see Gabriel eating at the counter. Gabriel watches them as they walk towards him and then Chin calls out his name. He takes off running and Chin chases him through the restaurant and out the back door. He turns to run down an alley when Kono bashes him with a garbage can lid.
In the interrogation room, Gabriel tells them his son has leukemia. He missed half of his son's life being locked up and when he gets out he can't pay his son's hospital bill, what kind of father does that make him? Kono asks if that's why he stole from the Fallons and Gabriel tells them he got another job driving the airport shuttle and was going to put the money back when he got paid. Chin adds they found out before he could and if they reported it he'd go back to prison so he decided to kill them. Gabriel is confused and tells them he doesn't know what sarin is. Chin shows him a picture of the victim and Gabriel recognizes him and tells them he knew him from the shelter with a free clinic and he was a nice guy. Even with his own problems he would always ask about Gabriel's son. He knew he was having a hard time so he told him about the rock. Chin asks what rock. Kono goes to the house and finds the rock by one of the plants. She shakes it and opens it to find a key.
At headquarters Chin tells Steve and Jenna that Gabriel's story checks out as the victim was staying at the shelter off and on for two months, the same days Gabriel's son was getting treatment. Steve adds if he had planted sarin in the milk, he wouldn't invite a hungry homeless man to stay over. Jenna laments they lost their only suspect and Kono enters and tells them they may have another as she pulled a fresh print off the key. It belongs to Elliot Connor, Jeff Fallon's brother-in-law and a COO of Jeff's company. There have been rumors about friction between Jeff and Elliot at their board meetings. Steve tells Chin and Kono to go talk to Jeff while he heads off to see Elliot.
Kono goes into Chin's office to find him finishing up a heated phone call. He tells her he returned the money to IA as it was the only way to keep their uncle out of prison. Kono asks where he came up with the money and Chin tells her he got a loan by bartering his house. Kono tells him when IA runs the serial numbers they'll figure out it's not the money Chin allegedly stole. Chin tells her they can't because the inventory numbers in the asset forfeiture locker went missing years ago. Kono is upset with Chin's recent actions and tells him off.
Steve and Jenna go to Jeff Fallon's company and they ask his assistant if they can speak to Elliot Connor. She hesitates and tells them he's in a meeting. Steve flashes his badge and tells her to free him up. Elliot tells them the rumors about friction between him and Jeff is not true, they've just had some disagreements. Jenna asks what kind of disagreements and Elliot tells them they had differing opinions on marketing strategy, but working with his brother in law they can always work it out as they're family. Steve asks Elliot if the Fallons ever have him check on the house and when he answers no Steve asks why they found his print on the spare key. Elliot thinks and tells them he dropped off some papers at the house for Jeff once but Jenna can tell he's lying. She tells him he shifted in his chair and touched his face when answering the question and added unnecessary details to clinch his answer. Steve smirks at Jenna's evisceration of Elliot and asks Elliot if Jeff knows he uses the house to cheat on his sister. Elliot adamantly denies it and Jenna catches him on using mock outrage. Steve adds there's a tan line on his finger like he forgot to put the wedding ring back on after his last hookup. Steve tells him he thinks either he's using the house for an affair or he was trying to kill his partner. Elliot admits they haven't used the house in months, not since Jeff confronted him about it and that's why there has been friction between him and Jeff. Jenna points out he's using his house to cheat on his sister, he has reason to be upset with him. Elliot tells them that Jeff told him to break it off with Chloe and he's going to, he just hasn't yet. Steve tells them they'll need to talk to Chloe and Elliot tells them she's right outside and they see he's having an affair with his secretary. "That's original." Jenna remarks.
In the hospital, Grace is playing Angry Birds on her phone and attempting to explain it to Danny when Kamekona joins them bearing food. They have a discussion about fried chicken. Danny tells Grace to put her headphones on and asks Kamekona about Sang Min, but Kamekona didn't even know he was back. Danny asks him to make a couple calls and Kamekona reluctantly agrees. He asks to use Danny's phone and Danny makes him wipe of his greasy fingers from the fried chicken.
In headquarters, Kono tells Steve and Jenna that the CIA contact has matched the sarin to a strand that was used in a botched Chechen terrorist attempt on Moscow five years ago. The Russian FSB identified the supplier as Mikhail Yursky. Yursky arrived in Hawaii five days ago under a fake passport and the timeline matches the murder. Jenna declares that's their killer and Steve adds now all they have to do is find him.
S.W.A.T, Steve and Chin burst in on Yursky's hotel room but he's not there. Kono and Jenna are at headquarters and Jenna tells him that Yursky purchased a flight to Borneo that leaves tonight. Kono tells them he's an ATM at Ventral Pacific Bank right now and brings his image up on the screen. She denies his transaction and he tries his card again when he sees Steve and Chin in their car coming down the street. Yursky gets back in his car and Steve and Chin block him from the front so he backs up and rear ends another car but keeps going. Steve and Chin chase him down streets and over sidewalks when he runs into a van backing up. Yursky gets his gun out of the glove compartment and a canister and runs out of the car. Steve and Chin chase him on foot down an alley and he shoots at them. Yursky climbs over a fence, but just as he reaches the top, he starts convulsing. Steve realizes he's been exposed and tells Chin to call an ambulance. Chin exits the alley as Steve tells him he's dead and then need the ME and HazMat. They look in Yursky's trunk and find more canisters.
In the hospital Danny and Grace are watching Spongebob Squarepants when Rachel enters. Grace tells her that Danno's not sick anymore and Rachel tells her she's very happy to see that and squeezes Danny's hand. Danny tells her she didn't have to come and she retorts she did too. He thanks her and she kisses his forehead.
Steve and Chin are back in headquarters and Chin has heard from HazMat who told him the canisters in Yursky's car were empty with no signs of toxins. Steve points out he was about to leave the island with them and Chin adds it doesn't make sense he would target two innocent people from the mainland. They see Kono who doesn't look to happy. Steve asks what's going on and they find they have a visitor. The visitor is David Akahoshi from IA who came to return the money to Chin. He tells them that the serial numbers on them don't match the stolen money, ergo he's trying to cover for his uncle. Chin protests those inventory numbers disappeared years ago, but Akahoshi tells Chin IA made it up that the inventory numbers disappeared as they were waiting for Chin to slip up and spend the money. But that never happened, because he never stole it. Kono interjects that this has gone too far. Chin tells Akahoshi to let it go as they got their money back but Akahoshi can't do that. He knows Chin was just trying to protect his family, so he's not going to charge him with obstruction of justice and he's not even going to ask where Chin got this money. But Keako stole from the HPD and the people of Hawaii and he's going to have to answer for that. Akahoshi leaves and Steve tells Chin he is going to ask where he got the money from. Chin tells him about the deal he cut with Marcum and Steve tells him to get his house back and since he did this for his uncle and doesn't have to explain anything else to him. Jenna enters and tells them they need to see something.
She tells them any chemist can synthesize sarin, the trick is not killing yourself in the process. She shows them the36 canisters found in Yursky's trunk were made by the Fallon's company. Kono clarifies Yurksy gets his canisters from them and his sarin ends up in the Fallon's house. Jenna also shows them he called Elliot Connor the day he landed.
Steve and Chin go back to the Fallon's company and show Elliot the picture of Yursky. Elliot doesn't recognize him and Chin points out he called the other day three times. Elliot denies speaking to him despite the fact the calls came in to his direct line. Steve tells him Yursky wanted a product of his companies and he wanted to get his brother-in-law out of the way so he could run the company, which would be conspiracy to solicit murder. Elliot asks when the calls were and when Chin tells him on Thursday the 13th. Elliot informs them that he was on the big island and didn't back until Saturday, adding that they can check with the hotel. They asked who else has access to the phone and he tells them the only one who takes his calls is his secretary. Chin and Steve go out to find she's gone.
As Chin and Steve enter the parking garage, Chin is on the phone having the entire building locked down and telling them to detain Chloe Ballantine. A car comes out of nowhere and Steve has to jump s out of the way. He shoots at the tires and the car crashes into a dumpster. Steve runs to her window and tells Chloe to put her hands on the wheel.
In the interrogation room, Chloe tells them that Elliot wanted to be with her and they were happy together. Chin clarifies she couldn't handle it when Jeff told them to end the affair. Steve asks how she got mixed up with Yursky and she doesn't answer. He makes her look at the picture of the victim and tells her he had a daughter and grandchildren and is now dead because of her. Chloe tells them Yursky contacted the company looking for a product they make, but the company refused the purchase order after doing a background check on him. A couple days later, Yursky called the office and offered her $20,000. All she had to do was fake some invoices. Steve guesses instead of taking the money, she asked him to commit murder. Chloe bluntly answers, "No. I took the cash too." Chin asks why she used sarin as there are many easier ways to kill a person and Steve guesses that's why. He goes on that it's big and bold and she thought the cops would think it was a terrorist attack and they'd be busy with the mass panic they wouldn't think of murder. Steve challenges that she never really loved Elliot and he was a payday to her and Jeff and Sheila were the only thing stopping her from cashing in. Chin adds with Jeff out of the way, Elliot could take control of the company and he and Chloe could be together. Chloe protests he was going to divorce his wife and marry her. But good was that without money? Chin tells her now she has nothing. Steve adds that's a shame as she could use that money now to pay for a lawyer. They walk out of the room, leaving her alone.
In the hospital, Grace has fallen asleep in a chair and Rachel has fallen asleep on Danny's chest. Danny wakes up to see Steve there, raising an eyebrow at the situation. Danny holds a finger to his lips for Steve to stay quiet but can't hide the huge grin on his face and Steve smiles back at him. Steve leaves with an amused look and Danny snuggles back into Rachel.
Back at headquarters, Steve opens another mysterious envelope and finds a medal inside. Jenna comes in and he shows her it's called the Order of the Rising Sun and was awarded to a Japanese pilot who bombed Pearl Harbor. Jenna asks how it ended up in his father's toolbox, but Steve doesn't know. He tells her his grandfather served at Pearl and was killed in the battle or Arizona. Maybe the person who mailed his father the postcards from Japan mailed him this. Jenna asks why he's receiving it now as everything else has some connection to Wo Fat. Steve answers, "I'll know when I find him".
Rachel pulls up outside headquarters and Danny gets out of the car. Grace asks if he can go to the beach with them and Danny tells her he'd love to, but he has to go work. He hugs her and thanks her for taking care of him in the hospital. Rachel tells him she's grateful this happened as she was looking for any excuse to come home. She tells him that her and Stan only went away to see if they could save their marriage but she doesn't think there's anything left to save. Danny tells her that's good and they kiss on the cheek and hug each other tightly. Rachel gets back in the car but as she drives away she keeps an eye on Danny in her side view mirror with a smile on her face. Danny watches the car drive off with a smile of his own.
Danny enters headquarters and is greeted warmly by Kono and Chin. Danny thanks Jenna and hugs her. Steve comes out of his office and sidebars Danny. He asks about him and Rachel and Danny admit it's been going on for a little while. Steve asks if he was ever going to tell him and Danny tells him they've gotten close and Steve teases him for getting soft from one near death experience. They hug it out, but the moment is short lived when Sang Min walks through the door. They all draw their guns but Sang Min shares that Wo Fat told him he's not going to rest until he's dead. And then he will come for Steve next.