Peter uses his Observer powers to track the timelines of Captain Windmark's top lieutenants, Royce and Mueller. He's adapting, but the tech is taking a physical toll. Plus, Olivia suspects he's hiding something.
According to Walter's fifth videotape, they need two of those cylindrical beacons the Observers use to locate different points in space/time (first seen in "The Arrival," Season 1, Episode 4). William Bell stashed a pair at an old Kelvin Genetics lab nearby, but now the entrance is buried under rubble, so they turn to Nina Sharp for help.
Before the invasion, Bell offered to help Walter stop the Observers. But he ultimately betrayed the team to gain favor with them. When the Observers found Walter, Astrid, Peter and Bell, the others realized Bell had exposed them. That's when Walter ambered everyone. And that's why Walter removed Bell's hand from the amber: They need his print to open the storage unit.
Outside the Ministry of Science, Nina and Olivia have an emotional reunion. Nina provides a matter-sublimation device that instantly converts solids into gas. They'll have to move quickly after using it, because the Observers will detect the atmospheric change.
Peter skips the Nina visit. Instead, he enlists Anil to switch a briefcase with Royce the Observer, but Anil can't make the trade due to a variable Peter had missed. Peter swaps the cases himself.
Privately, Walter asks Nina if she still sees any of the man he was before in him now. She says no. He fears turning into his old self but believes Peter's love will help him hold on. Nina reminds him that William turned on them, despite her efforts to hold onto him. Walter snaps that Bell never really loved her -- he never loved anyone except himself! Nina changes her mind; the Walter she knows would never say anything that cruel.
Royce meets with other top Observers. The decoy briefcase explodes, killing them with the flesh-dissolving toxin Peter took from Walter's cold storage (see "Pilot," Season 1, Episode 1).
Peter re-joins the team, and they use the sublimation device. Bell's hand gets them into the storage room, where Walter is outraged by yet another betrayal: Bell stole his David Bowie album!
Walter struggles to open Bell's safe, but Peter calmly helps him recall the combination (5-20-10). They find no cylinders, just an old photo of Nina . . . and a mysterious disk. When Peter accidentally activates it, the cylinders drill up out of the floor.
Later, a chagrined Walter gives Nina the photo and apologizes. Bell did love her. But it wasn't enough. Now he's scared. He begs her to remove the pieces of his brain again. It's the only way to hold onto the man he's become.
Olivia finds Peter working on his timelines. His detached tone resembles an Observer's as he explains how he killed the lieutenants and admits to implanting the chip in himself. It's the only way to win.
Peter declares they're going to avenge Etta. Windmark is next. He turns back to his work . . . and Olivia backs away.
Alone in his dark lab, Walter plays Bowie's "The Man Who Sold the World." Alone in his dark room, Peter tracks Windmark. He brushes his fingers through his hair . . . and a handful comes out.