In Chinatown, a soaking wet man runs to a woman and asks where Ping-On Street is. She nervously points it out to him and he runs ahead and buzzes at a door. A doctor, Ming Che, lets him in and the wet man says that he's sure that everyone else is dead. Ming Che says to wait and see and gets the wet man to a cot. The man complains about his stomach and begs for help. Ming Che puts on a pair of rubber gloves and removes a large pair of cutting shears, and then cuts open the suffering man's shirt. Something is bulging beneath the skin and crawling out of the man's mouth, and Ming Che grabs it with the shears.
At the Dorchester Bay Inlet, Peter arrives late and Walter comes separately in a taxi. Peter explains that Walter is practicing independence. He comes up and initially ignores Peter, and then notes that Peter was following him and doesn't trust him. They go to examine the site. A Chinese merchant ship registered in Hong Kong ran aground four hours ago and caught fire. At least 27 passengers on board tried to swim ashore but died. Upon examination, they realized that parasitical creatures burst out of their mouths. Walter confirms they're similar to ones found in livestock, but never that large. He pulls one out of a victim's mouth and has it transported back to the lab. The search teams find one woman who is still breathing and Walter tries to get her to treatment.
Ming Che is removing parasites from a number of corpses and then goes to talk to another survivor, telling him he's the first to arrive.
Peter and Olivia meet with Tao Chen from the Chinese consulate. He tells them the woman, Mei Lin, has been awake for an hour and lived outside of Hong Kong. The doctors have determined that she wasn't infected. Mei Lin explains that she came to America with the others for a better life, and is worried they'll arrest her. Peter, speaking Cantonese, assures her they're to help. The woman explains they were all given medicine for sea sickness, but she came from a fishing family and didn't take any. Mei Lin asks them to help her find her husband and daughter, and was told they were two days behind on another boat. They figure they have maybe 36 hours before the second boat lands.
At the lab, Astrid and Walter examine the parasitic worms and Olivia and Peter arrive to tell Walter about the capsule medicine they were given. Walter figures that the bodies provide warmth and food, enabling the parasites to grow, and the larvae of the parasite could have fit in a medicine capsule. As they wonder why someone would do that, Broyles calls to tell Olivia that the Coast Guard found a manifest that they traced to a local shipyard and arrested an owner.
Ming Che cuts open one of the parasitic worms, removes an organ, and puts it in a beaker of liquid with other similar organs. He then wraps up some powder and puts a label on it.
Broyles explains that the man they arrested was burning all the papers when they captured him. Peter explains that the man is a Sun Hong Triad gang member and won't speak as a matter of honor. His triad is known for smuggling heroin, and Peter suggests that the worms secrete opium. In the interrogation room, the man removes a hidden blade and cuts his own throat before they can stop him.
At the lab, Walter and Astrid take a blood sample from one of the creatures, but it grabs him and starts feeding on him. Walter admits that it's rather pleasant and Astrid finally gets it off. Broyles arrives and asks for Olivia, and they direct him to the back where she's with Peter. They're trying to figure out who would buy the drug. Broyles gives them the files on the Sun Hong Triad and its local leader, Jon Su. Su hides behind a number of shell companies, and they have 20 hours before the second ship arrives. The Coast Guard has over two hundred ships to search. Olivia spots a wire transfer from a bank in Boston to the Beijing Executive Construction company. The transfer corresponds to when the ship left China.
Peter and Olivia go to the address and find a private home. A teenager, Matt Jarvis, answers the door and reluctantly lets them in. His mother Elizabeth explains that she paid $500,000 to Beijing Construction on the advice of her attorney. Her husband died just after Matt was born. Peter looks around and notices hand sanitizer bottles all over the place, an air filtering system, and hermetically sealed windows. Mrs. Jarvis insists she knew nothing and gives them the name of her attorney. Outside, Peter asks Olivia to drop him at the lab while she goes to the hospital to talk to Mei Lin again
At the lab, Walter is putting on sneakers. Peter asks him if there might be a connection between the parasitic worms and a germophobe. Peter notes the bite marks on Walter's arm, but Walter insists that he's okay and his white cell count is up. He explains that hookworms are used for the treatment of chronic asthma in China. The parasites are a new species, bio-engineered from the hookworms, and could only grow in a human being. The creatures' lymph glands secrete an immune-boosting enzyme that could be used if someone was germophobic. Walter puts on a suit and plans to go to Chinatown to check on the herbalists who sell the hookworms for treatment of asthma. When Peter offers to do it, Walter insists he can do it and tells his son not to follow him… once he gets change for the bus fare.
Olivia shows Mei Lin photos of the Triad tattoos and she recognizes them from the crew that put her on the boat. She repeats her plea for Olivia to find her husband and daughter.
At sea, Mei Lin's daughter complains that she isn't feeling well. She notices that one man's stomach is twitching.
Olivia returns to the lab and Peter tells her that Walter is out and Astrid is watching him. He tells her what he saw at the Jarvis house and figures that Elizabeth is sick. Olivia wants to get a warrant to search the house, but Peter has a better idea. They go back there and talk to Matt, but he refuses to answer any questions to the FBI. Peter admits he's not an FBI agent and offers to play basketball while Olivia goes to get a warrant. As they shoot, Peter explains he was raised by his mother and can understand the fear of losing her. However, he warns that the people Elizabeth gave money to are scary people and they'll try to cover their tracks. Matt explains that the treatments are for him.
Walter wanders Chinatown looking for the herbalists. He finally notices Astrid following him and doesn't believe her when she claims it's a coincidence that she's there. Walter says that he wants to be alone and live his life with dignity and self-respect. Astrid apologizes and he offers to let her go with him as a friend, not a supervisor.
Olivia and Peter explain to Broyles that Matt suffers from an immune deficiency, and the people responsible insert the medicine into his spleen once a month. Matt is due for surgery the same time the boat is arriving and the doctor treating him is Dr. Ming Che.
Walter finds Ming Che's shop and buys some hookworms from him, while commenting he has worms that are 4' long. Once they leave, Ming Che makes a call to the Triad. When Astrid takes her eyes off Walter for a moment, a Triad member abducts him. She calls Olivia and Peter and explains what happened, but Peter figures that Walter ditched her to prove a point. Astrid goes back to the lab but there's no sign of Walter. She does find two Triad members taking the worms.
Walter tries to call Peter but keeps calling wrong numbers. He finally runs out of change and admits to a woman, Fae, that he's out of bus fare and can't remember the correct phone number sequence. The woman has no idea what he's saying.
Peter and Olivia return to the lab and find Astrid, unconscious. When they wake her up, she tells them what happened and they figure the men followed her back from Chinatown. Peter gets a call from Chinatown with news that Walter is okay. He goes to pick up Walter while Olivia goes to talk to Elizabeth. She tells her that Matt told them everything but Elizabeth insists that she can't tell her anything. Elizabeth insists that Matt won't live without the treatment but Olivia informs her that Mei Lin's daughter is at risk.
Peter goes to Fae's apartment and Walter explains they kept calling the numbers until they found the right sequence. He doesn't remember that Peter gave him the number and put it in his pocket. Walter thanks Fae and they leave, and Walter shows Peter the advertisement for Ming Che's, the only shop he went to with Astrid, and that he mentioned having 4' worms. Peter calls Olivia, who is at the docks. Elizabeth gave them a phone number and they tracked the ship-to-shore calls to the boat. She's preparing a team to board it when it arrives.
The FBI assault teams board the ship and start searching. Olivia finds crude beds and a child's toy, but no trafficked people. They realize they arrived too late.
Peter goes to Ming Che's address and sees the Tong members unloading the trafficked people. Olivia calls and Peter gives them the address and tells her what they're seeing. He then tells Walter to stay in the car and assures him he'll be careful. Peter picks the front door's lock and gets inside. He finds Ming Che operating on another passenger and attacks the doctor, but a Triad member knocks him out. They strap him to a table and Ming Che prepares to insert a parasite worm unless Peter tells them who else knows.
Olivia and the assault team pull up outside and Ming Che places the worm in Peter's mouth. Before they can force him to swallow, the FBI comes in. Ming Che goes for a gun and Olivia is forced to gun him down, and Peter spits out the worm.
Walter returns to the lab and blames himself for what happened to Astrid. She starts to cry and they hug.
At the hospital, the doctors do what they can and believe that most of the surviving passengers, including Mei Lin's family, will pull through. Given the number of passengers, they figure that Ming Che was treating other patients, and have found the records on six other individuals with conditions like Matt. Further, they've determined that "Jon Su" was a false front that Ming Che used for his operation. Olivia goes to see Mei Lin, who is with her husband and daughter, and gives the girl her butterfly toy back.
Later, Peter is sleeping at home when Walter wakes him up and explains that he wants to live like a man, not a child. However, he admits that there are times when he gets lost, and has implanted a tracking device in his neck. Walter gives Peter the transponder and lets him get back to sleep.