Walter attempts to transfer William Bell's consciousness from Olivia to a new host, but it doesn't work. Shortly after, Olivia resurfaces, wracked by seizures, and begs Peter for help. In the hospital, Bell regains control. At some point, he explains, the brain can no longer accommodate two consciousnesses. With the rats he tested decades ago, the host's consciousness just . . . went away. Right now, Bell's consciousness is keeping Olivia's body alive. In another day, she'll be gone forever.
Walter and Bell brainstorm a solution, involving their old Whole Brain Emulation experiment: They're gonna put Belly into a computer! They'll enter Olivia's mind, like that time when she and Agent Scott shared the dream state, and guide her out. Peter reminds Walter that dream-state thing involved high doses of hallucinogens. Of course, say Walter and Bell: LSD! And since Peter knows Olivia's mind best, they'll need him as a guide.
Astrid mixes up 2,000 milligrams of LSD and puts it on some sugar cubes with an accelerant, so the drug will take effect quickly. Broyles arrives as Astrid is preparing a sedative for the dosed trio. Peter stares at his head. "You're bald," he marvels. He tells Astrid he thinks Broyles is an Observer. She tells Walter that they're ready.
Astrid and Broyles watch Peter drift away under sedation . . .
. . . and then Peter's in Olivia's mind. He's on a busy New York City street, dressed in black like everyone else, except tweedy Walter. But where's Bell? Walter says everyone's dressed like Olivia because Cortexiphan subjects have a strong desire to blend in; they were designed that way. Before Peter can find out what THAT means, an SOS flashes from the top floor of the Twin Towers. It's Olivia!
Broyles watches Astrid prepare the computer for Bell's consciousness. He gazes at a red licorice twist, rhapsodizing about how swirly it is. Astrid realizes he touched the sugar-cube tray. Broyles is dosed!
Walter spots Olivia's evil stepfather, and then a crowd eyes them suspiciously. Run! At the Towers, Nina tries to kill them! But Olivia isn't in Bell's office, only Bell. He's in his original form - except he's a cartoon. He deceived them because he feared they wouldn't come just for him. The Bishops enter the office, and they become cartoons too! A thought balloon appears above Walter's head: "How wonderful!"
Olivia's mind is attacking them because she doesn't know Bell's consciousness entered hers. Her fears are causing the siege. They go to the roof, where a zeppelin floats overhead. Peter realizes that she's hiding, probably in Jacksonville, the last place anyone would look. Suddenly, the stairwell door bursts open - AND ZOMBIES CHARGE OUT! The three men escape in the zeppelin.
Broyles is tripping hard. Astrid gently explains he's just feeling the effects of the LSD. He laughs and says he knows it followed him. What did? "Death," says Broyles. "I saw death, all of it, and it was ME." Wide-eyed, he asks Astrid to hold his hand. She does. Poor Broyles. He never should've looked inside that van holding dead Broyles from the Other Side.
Onboard the zeppelin, Walter says he should have known Bell had no intention of dying that day, when they were Over There rescuing Peter. Bell said goodbye to Walter, and Bell hates goodbyes. Walter plaintively says that he needs Bell's help with Peter and the machine. Bell replies that, unlike when they were young, Walter now possesses the wisdom of humility. He will make the right decisions.
Meanwhile, someone cuts the fuel cable! The engine room is locked from outside. Peter opens it, and a strange man bursts out, leveling a flare gun and asking his name. Walter doesn't recognize the guy either. The man grabs a parachute, blasts a hole in the zeppelin, and leaps. The decompression pulls Walter out too! He falls earthward . . .
. . . and wakes up in the lab, in the flesh. He's OK, but Bell and Peter are on their own.
Bell and Peter go to the military base, Olivia's childhood home. She once told Peter that the last time she felt normal was the day before she met Bell and Walter. That's where she's hiding. Peter steps through the door . . .
. . . and he's not a cartoon anymore. Olivia's there. Behind her is little Olivia, with her parents. He looks into Olivia's eyes and knows she isn't real. Little Olivia - the real Olivia - says she knew the real Peter would recognize her. Suddenly, it's Christmas, and Olivia's stepdad chases them! Outside, it's all cartoony again. Peter shoves little Olivia away from a speeding Humvee, and THUMP! He's knocked back into full-fleshed reality. It's up to Belly now.
After little Olivia confronts her stepdad, stops the attack, and turns into a grown-up, Bell realizes she let her fears overwhelm her, but she fought back. And now she knows she's as strong as he and Walter always believed. Bell says to tell Walter he knew the dog wouldn't hunt. Olivia wakes up, whole again. Nearby, Broyles is blowing bubbles. Walter sees one consciousness on the Bell/Olivia monitor. Triumph!
But Bell's not in the computer. William's gone. Walter hears Bell's message; it's what Bell said when he knew something wouldn't work. He knew he wasn't coming out . . . and he hates goodbyes. Walter sadly retreats to his office.
Olivia tells Peter that she's not afraid to move forward anymore. Peter looks at a sketch she made. He saw this guy, locked in a zeppelin in her mind. Who is he? Olivia shrugs. She doesn't know. "But I think that he's the man who's gonna kill me."