“Subject 13” is episode fifteen of season three of Fringe.
The episode starts with someone walking across the frozen Reiden Lake. Elizabeth Bishop calls for Peter, and finds a note for him saying he’s going home. She finds him on the lake, with a rope tied around him and to a concrete block, which he uses to break the ice. She goes in and manages to get him; he says she’s not his mother and he wants to go home.
The 80s-style credits from “Peter” reoccur.
Walter is with the children at Jacksonville Family Daycare, including a young Olivia, when he’s interrupted. His wife is on the phone. And she’s here in Jacksonville. Walter finishes up the session.
Walter finds Elizabeth and Peter and Peter is saying this isn’t his world, stating the differences. He’s told that he was very sick for a long time and his memories are mixed up. Peter says they aren’t his parents and he wants to go home.
Elizabeth tells Walter she can’t keep this up. It was supposed to be temporary; they were supposed to get Peter back home and it’s been six months. Walter says it isn’t as simple as thought. Elizabeth asks why he can’t use a device like the last one. Because the texture of the universe was changed by his crossing, and it can’t take any more damage. Elizabeth says Peter can’t either. Peter is like their son but not their son; Walter saved his life, Elizabeth just tries to keep him alive. Walter thinks the children are key to crossing over and getting Peter safely home.
Olivia is reading when someone orders her to go to bed, and runs after her when she flees. She briefly ends up in a field, hearing and seeing something surprising, before returning and being hit.
Elizabeth is making breakfast for Peter. He knows she’s not his mother and he thinks he, meaning Walter, makes her say that Peter’s memories are confused from his illness. Walter stole him from the world at the bottom of the lake. Peter knows he sounds crazy, but he isn’t.
Walter is talking to his assistant, Ashley, at Jacksonville. What they’re doing isn’t working and he wants to try testing the children individually. Then notices Olivia and asks why she’s there. Walter goes to speak to her; she’s got a hurt eye that she told Ashley, and tells Walter, came from falling whilst running in her house. Walter tries to get Olivia to tell him the truth. Then asks what she’s drawing. Ab airship. He asks if she saw it last night when she fell. Because she clearly crossed over.
Elizabeth is driving with Peter, past a field of white tulips, which she talks about. What would he want to do? Go home. They head to a toy ship and Elizabeth follows Peter around as he looks at toys. Finally, when he picks up an aeroplane, it seems they make a connection.
Walter tells Ashley he suspects Olivia’s crossing over was a result of extreme emotional response; he wants to design a series of experiments around that. Elizabeth and Peter arrive; Peter notices Olivia and they seem to make a connection. Neither seem to remember meeting the other later, but Peter’s and Olivia’s memories from that time are known to be missing bits. Walter tells Elizabeth he thinks Olivia crossed over and if he’s right, they can finally take Peter home.
Walter writes to William Bell about Subject 13 – Olivia. He thinks she’s crossed over to the other side during a heightened emotional state. The recordings are on Walter’s new Betamax – should have bought VHS – and perhaps Bell can see something Walter cannot. The videos show Walter doing a series of experiments designed to elicit different emotional responses. At one point Peter watches, then Walter decides they won’t watch Jaws. He thinks fear by itself is not the answer and leaves Olivia alone. She plays around until the lights go out. The door is locked. She hears someone call ‘Over here’ then the lights come on and there’s a bloody boy playing dead on the floor. And fire flames out from Olivia.
All of this episode is set in the past, and most of it in the main universe. However, Walternate and AltElizabeth are seen too, and Walternate in particular is not doing well. He’s had theory after theory about what happened. And it’s revealed how he finally discovered what actually happened.