Peter watches an Observer stronghold and uses his enhanced abilities to see the most probable futures. He plots the actions of three Observers. Once he's done predicting the future, Peter steps out into the middle of traffic and stands there, watching the Observers in their car. Once he's delayed traffic sufficiently, Peter walks away and time proceeds as he observed it. Satisfied, Peter watches as the Observer he predicted meeting his comrades misses them because they were delayed in traffic. Peter then calls Alin and tells him that he's going to need his help.
Later, Peter returns to the Harvard lab with the supplies he claimed he was going to get. When Olivia wonders why he slipped out without telling her that morning, Peter says that he didn't want to wake her. She points out that he hasn't been sleeping but Peter says that he's fine. A loyalist patrol pulls up outside and everyone freezes. Once the security men finish giving the building a brief lookover, they leave and Walter and Astrid extract another videotape from the amber.
The team watches Walter's recorded message where he explains that they need to get two of the Observer's drill cylinders that they use to home in on points in time. The directions lead to Kelvin Genetics, which Bell owned and performed experiments with Walter. Walter remembers that Bell kept a storage chamber there and that it has a palmprint security scanner. Astrid realizes that was why Walter cut off Bell's hand when Etta first freed them. They remember that Bell initially claimed to work with them but then betrayed them to the Observers. Walter called in Peter and Olivia, but they had no choice but to enter the amber when Bell summoned the Observers. Astrid and Walter go back to work cutting Bell's hand out of the amber.
After extracting the hand, the Fringe team drives to the genetics facility, only to discover that the building has collapsed. Peter spots a door using his enhanced senses, but dismisses it as his engineering instincts when Olivia wonders how he spotted the door. She points out that Peter is bleeding from his ear and Walter examines his son. He doesn't find anything wrong and Olivia suggests that the concussion from the antimatter baton they used to close the wormhole might have caused the damage. Astrid suggests that they use another baton to clear away the rubble, but Walter says that it could collapse what's left of the building onto the underground storage vault. Anil calls and Peter talks to him privately. Anil says that the Observer Peter told him to watch, Royce, never left his briefcase in the park so he never got the chance to put in the substitute. Peter says that's impossible and tells Anil to meet with him.
As Peter goes to the car, the others decide to meet with Nina and see if she can use her position in the Ministry of Science to get them technology that will let them clear the rubble. Peter says that Anil called and asked for a meeting, but wouldn't say what over an open channel. When Olivia looks at him suspiciously, Peter tells her that there's nothing to worry about.
In Brooklyn, Nina gets an unmarked letter asking her to meet with the team. She goes to the river and finds Olivia waiting for her. Nina offers her condolences on Etta's death and admits that she didn't contact them earlier for fear of blowing her cover as a member o the resistance. Olivia explains that they had to meet with her and takes her to Astrid and Walter. When they describe what they need, Nina informs them that the Observers have a device that sublimated solid matter, transforming it into vapor. She tells them to meet her outside of the Ministry in ten minutes.
The team meets Nina in the Ministry parking garage and Astrid and Olivia wait in the van while Walter meets with Nina. Nina asks if he's angry with her because she told Simon and Etta how to implant the pieces of his brain that were restored. He admits that she did what she had to and asks if there are any changes in hie personality. Walter tells her that there have been and asks if Nina sees the man he was before in him. She admits that she doesn't and asks if he's worried, and Walter explains that the man he was before was consumed by ambition and pride and cared nothing for anyone else. He believes that Peter can keep him from changing back.
Peter meets Alin and has him go over everything that Royce did in the park. Alin describes how Royce talked to someone on his comm and Peter realizes that he didn't see that. Checking the probability paths, Peter confirms that Royce is going to a restaurant and takes the case from Alin. He tells Alin to meet for him outside of the Observer stronghold and call him when Royce and Mueller walk inside. Peter steals a car and starts to drive away, but is briefly overwhelmed by shooting pains.
Once he recovers, Peter drives to the restaurant, Cavatina, and has the coat check girl check it behind the counter with the others. She warns Peter to be careful since the Observers can read him at anytime, and Peter tells her to answer the phone. It rings a second later and the girl stares at him in surprise, but Peter says that it's just a trick. He then sits at the bar and waits.
As Walter meets with Nina, Astrid and Olivia wait in the van. Astrid asks how Olivia is doing and she admits that Peter isn't sleeping and is always going off somewhere on his own. She's sure that there's something that he isn't telling her, even though he always has a reasonable answer for where he's been. Astrid points out that Peter isolated himself after losing Etta but he's come a long way since then, but Olivia is worried that she's losing him again.
At Cavatina, Peter watches Royce as he comes in and checks his briefcase. Once the Observer sits down to eat, Peter goes over to the coat check girl. She gets a call from her boyfriend while Peter points at Royce's briefcase and the distracted girl gives it to him.
Hastings shows Astrid and Olivia how to use the sublimation devices. Meanwhile, Nina warns Walter that Peter's love may not be enough to stop Walter from reverting. She points out that Bell was arrogant as well and that her love for him didn't stop him. Walter coldly tells her that Bell never loved her and that she should know better, and Nina tells him that now she's seeing the man that Walter once was. Walter insists that Peter's love will save him and Nina hopes that it's true. Olivia comes over to tell them that they're ready, but warns Walter that the Observers will notice the atmospheric change and be on them within a few minutes. Nina wish them good luck and they leave in the van.
Alin watches the Observer stronghold as first Royce and then Mueller arrive. The two of them go up with the third Observer from the car and enter a briefing room. When Royce opens his case, it releases an explosive burst of gas, causing the Observers' flesh to melt.
Peter goes back to the storage facility as the others set up the sublimation device. Alin calls to tell Peter that his plan was a success and that dozens of Observers have come to investigate the explosion. Peter tells Anil to go home, hangs up, and tells Olivia that Anil had set up a meeting but missed it due to a checkpoint inspection. Olivia isn't entirely convinced but lets the matter drop as Walter activates the device and the rubble dissolves.
The team enters the ruined upper structure, powers up the scanner, and holds Bell's hand to it. Once inside they discover that Bell has an extensive collection of Fringe artifacts. Walter finds an old album of his that Bell stole. They search the room and find a special safe where Bell kept his most prized possessions. Walter figures that Bell used the same combination that he did for everything else, but he's unable to remember it. Astrid and Olivia warn that the Observers could arrive at any time and they should leave, but Peter tells Walter to relax and focus. His father gets the combination and opens the safe. The only things inside are a small metal disk, several folders, and a photograph of a much younger Nina.
Peter activates the disk and the building starts to vibrate. The two drill cylinders come up from beneath the floor and the team grabs them. Olivia warns her husband that the Observers could be outside but Peter walks out, unconcerned. He says that they should split up and that he'll take one and they'll take the others. As they leave, Walter tells Olivia that he needs to go somewhere first.
That night, Walter meets with Nina and tells her that they have the drill cylinders. He gives her the photograph and tells her that Bell kept it in his safe with his most valuable possessions. Walter admits that he was wrong and that Bell did love her... but it still wasn't enough to save him. He asks Nina to remove the pieces of his brain again, insisting that he won't go back to the way he was and that he can still do what is necessary without them.
When Olivia returns to the lab, she finds Peter hard at work sketching out the timelines of the three Observers that he killed. He explains that he was able to see the probable futures of each Observer and then use that information to make sure they were in the same place with his bio-weapon. He kept Royce from getting in the town car and missing his meeting with the other two Observers. When Olivia wonders what he's done, Peter explains that he put the Observer tech into his own brain. After he calculated the probable futures, he used the flesh-eating virus from Walter's cold storage vault to create a bomb. Olivia realizes that her husband plotted the entire thing so that the Observers wouldn't notice the atmospheric disturbance caused by the sublimation device. She tells him to stop but Peter speaks in chorus with her, predicting exactly what she will say. He tells her that he plans to kill their daughter's killer, Windmark, and goes back to work at the board, sketching out the Observer's probable future. Olivia backs out of the room and Peter ignores her.
Walter puts the cylinders with the other pieces of his plan that they've assembled. He listens briefly to the radio from the pocket universe but hears nothing. He then puts on his recovered album, "The Man Who Sold the World," as Astrid comes in. She tells him that they did good, gives him a peck on the cheek, and goes to bed. Walter turns up the music and lies down in the dark to listen to it.
Peter plots what he can see of Windmark's likely futures. He writes down that the Observer will get into an elevator.
Windmark leaves his office and takes the elevator down.
Peter tugs at his hair and it comes out of his scalp.