At Harvard, Astrid is awakened by the transistor radio the Fringe team found in the pocket universe. It's broadcasting a signal, over and over. Walter is behaving strangely.
Peter tells Olivia he had another one of those headaches. Astrid fills them in on the signal: the pattern repeats on a loop, but she can't decipher the code. Olivia wonders if the mysterious Donald is trying to contact them.
Walter has taken LSD. He hallucinates a flying pixie . . . and his long-dead lab assistant, Dr. Carla Warren, who was burned in the fire. He tells Peter he found some "Black Blotter" and took it because he's desperate to remember the plan to defeat the Observers before "he" - the Walter he was - completely takes over. When the plan is complete, Nina will help him remove the parts of his brain that were replaced. He doesn't want to lose his loved ones . . . or the man he has become.
The acid fuels a surreal moral drama in Walter's mind. Carla is an advocate for the old Walter. Another vision, of Nina Sharp from when she first tried to stop his younger self from crossing to the Other Side, represents his desire to remain the man he is now.
Anil helps them trace the signal, which leads Peter and Olivia to a decaying RV in the Connecticut woods. They find the bones of an Observer and two Loyalists. At the RV's wheel is another skeleton: Sam Weiss, whom Peter and Olivia remember from the other timeline. They discover that the signal originates from a nearby island.
Walter's internal struggle continues. Carla leads him to a hidden notebook, full of the dangerous ideas she came to burn the night she died. Nina and the pixie warn Walter off . . . but he can't resist looking.
Walter finds himself with the notebook in a cab outside Observer HQ, planning to impress the invaders with his amazing mind. He asks Carla why he's thinking of a black umbrella. When he again insists he's not the same man, she says, "You've been him longer than you've been you."
In reality, Walter's in the car at a dock. The team's taking a motorboat to the island. A boatful of Loyalist troops try to intercept, but Olivia and Peter take them out.
On the island, Carla is still with Walter. The team finds the Observer child living with a couple, longtime resistance supporters. Donald brought him there more than 20 years ago. The boy, whom they call Michael, doesn't speak and hasn't aged a day. When the husband demands the password Donald put into the coded message, Walter retreats into a psychedelic fantasy and steals a key from his sleeping, armor-clad alter ego. Emerging, he announces that the password is "black umbrella."
Correct.Back at the lab, Olivia asks if Michael knows her. He nods. Peter wonders how, since they met in a different timeline. Olivia says Observers experience time differently; maybe Michael does too.
Walter watches a mind-movie of his arrogant younger self, the real Carla's attempts to dissuade him, and his great mistake of crossing over. He burns the notebook in a glass bowl, but Carla says it's too late: he remembers all that he's capable of. Nina argues that Walter has to keep fighting. Walter sees that there's no notebook in the bowl, just fire. Again Carla says, "You've been him longer than you've been you."