Everyone looks uncomfortable as Astrid says that it wasn't the only reason. Walter doesn't know what she means, and Astrid explains that before the invasion, Walter had been talking with Bell, who said he wanted to help Walter defeat the Observers, that he realized how important it was: "And then he betrayed us to gain favor with them."
It all starts to come back to Walter, as he recounts a tale of another man, with an accent, who was torturing Walter and kept asking, "Is it safe?" Then he escaped and Bell helped him, but drove him right back to where he was being questioned. "He worked for them. He worked for them!"
Thank god Astrid has seen Marathon Man because she tells Walter that's where that's from. Peter's turn: He tells Walter that Walter called Peter to tell him it was time to implement the plan, but Walter needed Peter's help. Olivia chimes in to say Walter sent her to Grand Central Station to get the thought unifier. Peter says by the time he got to Walter and Astrid, Bell was with them, but then the Observers found them, and they realized the only way they could have known where they were was if Bell had told them. And that's when Walter ambered them. "So if Bell led the Observers to you, how can we be sure that the cylinders were even there to begin with?" asks Olivia. Fair point. Astrid suggests they open up the storage and find out, and a subdued Walter tells "Agnes" to fire up the laser so they can Bell's hand back out of the amber.
They leave the hand-retrieval off-camera and head to Kelvin Genetics, right off the water, but it's nothing more than bombed-out rubble now. Walter points out where he used to drop acid and watch ships come in from the bay. They're right by the east entrance, he says, and the yellow doors mentioned by Video Walter are here somewhere. Peter surveys the scene in his Observo-vision, displaying the underlying structure, including a doorframe. He points at it: "The doors are there," he says confidently. Olivia looks at him, surprised, and says, "Peter?" and he quickly covers by saying, "From an engineering standpoint, they'd have to be." But it's not his certainty that she's worried about: it's the blood trickling from his ear.
Walter examines the ear, Olivia asking if the blast from the RPG Peter fired into the Observers' shipping lane could have damaged his ear, but Peter says his hearing is fine. Walter says he wants to examine Peter as soon as they get back to lab.