Dr. West is about to do an MRI on Walter. He's scared and asks Peter what Dr. Paris did to him. Peter says they will find out. Olivia joins Peter as he waits for the test results. Peter knows that Walter wants to turn back the clock to before he went crazy. He's just sane enough to realize what he's lost. Olivia says that, based on what she knows, going crazy made Walter a better person. Peter still feels guilty for not visiting but Olivia assures him that he is more than making up for it now. Dr. West returns with the MRI. Walter had three pieces of his hippocampus removed - the part of the brain that stores memories. Peter realizes that the three other patients where storing bits of Walter's brain.
Walter is at home, trying to get over the drugs and the trauma of the MRI and results. He thinks he's ODing on Valium and wants to listen to a particular album to calm down. Astrid offers to go get it for him.
Peter is putting the pieces together - figuratively speaking. This particular scene is a crystal clear picture showing that Peter is definitely Walter's son - quick thinking, intuitive, and smarter than almost everyone else on the planet. Why remove pieces of Walter's memory? Because Walter literally had the memories of how to open the door between worlds removed. How could the bad guys use these pieces? By hooking them up to a brain that could interpret the data. And there is only one Walter Bishop. (This last point may not be true, but it is for now.)
Astrid gets a call from Peter, telling her to get back to Walter ASAP. But she is too late. Peter tells her not to worry. Walter implanted a subdermal transponder - they can track him that way. Unfortunately, the bad guys seemed to know about it and cut it out of Walter's neck. Peter and Olivia have no idea where Walter is.
Meanwhile, in Walter's old beach house, Newton is hooking up Walter to some kind of memory device. Since the pieces were removed, Walter has formed new pathways to his memories and Newton must hook up the pieces to the correct places in Walter's brain. In order to map Walter's brain, Newton uses images to form associations. Almost everything Walter sees - a young boy, a custard, a coffin - reminds him of Peter.
Peter's brain is working at full speed. He knows that almost everything Walter does is part of memory association (and now it makes sense why.) Peter thinks about the three other patients. He realizes all the clues are about him. When he was a boy he played with a little girl who wore a red dress. She lived across the street from his old house - 2828 Green Street. The bad guys have taken him to where Walter realized how to open the door.
The visual images aren't going fast enough so Newton forces the reconnect. When it hits... Walter's not Walter anymore. John Noble did a fantastic job of having a new character in an old body. The "old" Walter is a miserable, egotistical, scary ass and it's no wonder Peter didn't go to visit him. He's an evil bastard. Walter talks to Newton and asks how things are going on the Other Side. Newton tells him that a blight has killed off most of the vegetation. Newton orders Walter to tell him how he opened the door.
Curtis tells Newton that Peter and Olivia have arrived. Newton injects Walter with something and runs for it. Peter rushes for Walter, who is once again the Walter we know and love. Walter is distraught as he sees his bits of brain in the beakers. They are dying and they hold memories he can never get back. Peter tells his father that he doesn't need those memories.
Olivia checks out the rest of the house. She finds the owners tied and gagged in the back room. They tell her the bad guys went out the back door. Olivia flies out of the house and sees a vehicle pulling away. She fires into it, killing the driver. Then she shoots Curtis in the head. Newton stumbles from the back of the van.
Newton tells Olivia that he injected Walter with a neurotoxin which will kill him in 4 minutes. If Olivia lets him go, he will tell her how to save Walter. Olivia calls Peter, who confirms that Walter has collapsed. Olivia reluctantly agrees, tossing her phone to Newton and running back to the house. She gets Newton to tell her where to find the anti-toxin. Newton gloats that now he knows Olivia's weakness. But Walter recovers.
Broyles debriefs Olivia. She beats herself up over making a personal rather than a professional choice. She chose to let a bad guy go rather than sacrifice Walter. Broyles says that it was a logical choice. He's rather philosophical about the fact that they have more questions than answers. Olivia doesn't believe him. They got no answers. Broyles says that she gave the enemy a face and a name. They will have other chances to get him. But she had to save Walter - he is the only one who can help them with their fight and they are going to need him.
Peter talks Walter into getting another MRI. As Walter enters the machine, he flashes back to his meeting with Dr. Paris.
Dr. Paris is William Bell. Walter was aware that it was Bell who was about to perform surgery on him... and what the results would be. Bell promised Walter that the information he removed - how to open the door - would be kept safe. Walter was a willing participant to his own insanity.
[...] is unsuccessfully arguing with Walter, who won’t get out of the car and go into the supermarket. He’s afraid of being kidnapped again. Olivia calls with a case – a missing kid and three dead state troopers. Peter says that it [...]