At a U.S. army research headquarters, Walter tells several military generals that they must embrace the possibilities of science, and shows them a cell phone. He apologizes that Dr. Bell can't be there, and informs them that the telephone is from an alternate universe that possesses technology in advance of theirs. Walter admits that it's impossible to go there, but he was able to duplicate the cell phone. He takes them to the roof where his colleague, Dr. Carla Warren, has set up a "window" that allows them to see into the other world. They proceed with the demonstration and show them the Empire State Building in the other universe, where it is still used as a docking station for dirigibles.
Walter comes to see Olivia at her apartment and explains that she left him no choice but to come see her in person. She reluctantly invites him in and pours him a drink, and Walter asks if she's told Broyles what she knows about Peter's alternate universe origin. Walter shows her the window device that shows the other universe, and explains that nothing mattered after Peter suffered from a genetic illness. There was no hope on their side, so Walter looked into the other universe. He sought out the other Walter to see if he had a sick son who needed a cure, and discovered that he did.
At his lab, Walter watches through the window as his counterpart creates a cure for "his" Peter using cobalt and magnesium. However, it's unsuccessful and Walter checks his own research. His wife Elizabeth calls to ask him to come home. Walter agrees and when he arrives, Elizabeth tells him that Peter is worse and insists that he talk to his father. Walter puts on some music and reminds Elizabeth that he's spending all of his time trying to find a cure. He assures her he won't give up, and asks her not to doubt him. Walter then goes to see Peter, who is practicing finger-rolling a silver dollar. The boy asks Walter to keep it because it's his lucky coin, and says that he's not scared. Walter promises that nothing will happen to him and he'll soon be fine, and then tells him to get some rest. They embrace… and Peter dies.
Peter and Elizabeth bury their son, while Carla and Nina Sharp look on. As they leave, Nina apologizes for William, who couldn't be there because of an important meeting in Berlin. Back at home, Elizabeth is guilty because they never gave Peter a good life. Walter wonders if Peter knew that they loved him.
That night, Walter wakes up Elizabeth and tells her to come with him. He takes her to Peter's room, where he has set up the dimensional window, and shows her the other Peter. Walter explains he wanted her to take comfort in the fact that somewhere, Peter would have a happy life. Now he wants her to move on.
Walter continues to watch his counterpart and realizes how he's coming up with the compounds for the cure. The Observer, September, then arrives in the lab to talk to the alternate Walter. Walter watches as the cure briefly turns the proper color, but then reverts because it's unstable. The alternate Walter doesn't realize what happened, and a frustrated Walter smashes the dimensional window.
In the alternate dimension, September meets with his associates, August and December, and informs them that he made a mistake by meeting with the alternate Walter. Walter discovered him when he tried to witness the moment, and created a new set of probabilities. They tell him that he will need to restore the balance, and the opportunity will reveal itself to him.
At the lab, Carla discovers that Walter is working on the cure. He explains that he's working to save the alternate Peter, and plans to use the Casimir Effect to create a portal and take the cure through the wormhole. He's determined that the alternate Bishops are going to their lake house. Carla warns against it, noting that the wormhole will rupture the fabric of the universe. Walter dismisses her concerns as religious claptrap, but she insists that the energy required will destroy both universes. He refuses to let Peter die again, and concludes that the lake will absorb the excess energy. Walter then fires Carla and she leaves.
Carla goes to see Nina, who insists that Walter's proposal isn't possible. When Carla explains that Walter has already built a portal, Nina calls Dr. Bell.
At Reiden Lake, Walter arrives and checks his readings, and then takes his equipment out onto the lake. Carla and Nina arrive and try to stop him, but Walter points out that Bell has encouraged him to take this risk, and would be there if he thought there was any danger. Nina doesn't believe it, but Walter explains that Bell has only ever cared about improving his wealth and power. She tries to get through to him, but Walter ignores her and activates the portal. When he tries to go through, Nina grabs him but he pulls himself through . The portal shuts down, and Nina stares at her hand, now dimensionally unstable.
Walter discovers that the vial of the cure was broken in transit, and runs off into the night.
At the lake house, the alternate Peter is practicing the coin trick when his mother comes in to tell him to go to sleep. He gives her the lucky silver dollar so she can have it if she dies. Elizabeth promises him that he'll be fine. Once she leaves, Walter comes in and takes the alternate Peter's hand. Elizabeth comes in and Walter pretends to be his counterpart. He tells them that he has a cure but needs to take Peter to Cambridge right away. Walter talks to her privately and suggests that she stay there while he finishes the cure. When she balks, he asks her not to doubt him and Elizabeth agrees. She gives Peter his lucky coin back and hugs him, and then asks Walter to bring her son back. He promises and leaves with Peter in his arms.
Walter takes Peter out to the lake, and the boy quickly realizes that Walter isn't his father. Walter insists that he is, and promises to make him better. Walter activates the portal and they pass back to Walter's reality, but the ice breaks beneath them. Walter watches as the alternate Peter sinks into the waters beneath him. As Walter passes out, September swims down and rescues them both.
Walter wakes up in September's car. The Observer starts echoing Walter's words, and then explains that Nina and Carla left to seek medical attention. He warns that Peter won't live much longer unless Walter fixes him. September then pulls the car over and starts to get out. When Walter wonders why he intervened, September tells him that Peter is important and has to live. Walter takes the wheel and drives to the Cambridge lab.
At the lab, Walter tends to Peter and Carla arrives to help. Peter is recovering, and Walter says that he'll return Peter once the boy has stabilized. Once Carla leaves, Elizabeth comes in to see if he's okay. She sees the alternate Peter, and Walter explains he could only save him by bringing him to their universe. Elizabeth embraces him and starts crying, and Walter tries to explain that he must take her back. His wife just stares at him beseechingly.
Walter explains to Olivia that he couldn't bring himself to take Peter back despite his promises. That was the first breach of many, and it's all Walter's fault.