While getting high with Kevin the security guard, Walter looks at Massive Dynamic's CCTV monitors and asks what's in that room? No one ever goes there. Kevin says that's William Bell's office.
Two hours later, Walter wheels a stack of boxes into Nina's office: animal ESP, a morality detector, viral dreaming, powdered water - some of their best ideas! Why is he going through William's old research? Walter's feeling the pressure to protect this universe. Peter's happiness with Olivia (which is welcome news to Nina) is a constant reminder of what's at stake. He remembers how good he and Bell were together. If he goes through every detail in these files, maybe he can recall the way they used to work, back when anything was possible.
Two thieves at the Massachusetts Metal Depository climb down a building - but, weirdly, they're traveling upside down. One rights himself, zips into a pair of heavy rubber boots waiting on the ground, and flees. But the other, Koenig, is shot by a security guard before he can get into his ... and his body floats upward!
Peter continues his secret shapeshifter research. By daisy-chaining the memory disks, he's found previously undetected data, but can't break the encryption. When Olivia calls, he lies and says he's at the gym.
Soon they're meeting Broyles at the depository. Koenig's floating body is still tethered to his climbing ropes. They find a key card on him. Peter observes the thieves used the weighted boots for counterbalance. It appears that they took a large quantity of osmium, a metal twice as dense as lead but not very valuable.
Koenig's corpse floats in the Harvard lab. The weightlessness is wearing off. His key card has been traced to a Boston warehouse, so Olivia and Peter head out. En route, they play a game of "Full Disclosure": Tell the truth, whatever the question. Later, Koenig's body drops to the floor. It's too heavy to lift. Walter has Astrid test his blood for osmium, even though it shouldn't make him float.
The surviving thief, still wearing his heavy boots, tells a man named Dr. Krick about Koenig's death and says he feels ill. Krick asks if he got the stuff. Later, they meet at Krick's lab - in that Boston warehouse. The thief's eyes are bleeding. He asks why the effect isn't permanent, like Krick said, then dies before Krick can give him an injection. The doctor dodges Olivia and Peter, who find the thief's partially dissected corpse ... and a walk-in freezer containing more bodies. All had toxic levels of osmium in their blood - and suffered from muscular dystrophy.
At Harvard, Walter rants that the person behind this is altering the basic properties of osmium. Messing with the fundamental constants of our world! It could lead to chaos! Nina arrives, and Walter, desperately yearning for Bell's help, shows her a file on "soul magnets." The idea is to provide a dead person's soul with a host, someone who's had microscopic soul magnets inserted into their body. If they knew how to trigger the magnets, they could summon Belly's soul to its host! Nina is highly skeptical.
Meanwhile, Krick watches a wheelchair basketball game, cheering on a player named Michael, who turns out to be his son. Vince, another wheelchair-bound youth, watches wistfully nearby. Soon Krick's injecting Vince, who literally floats on air. Krick gives him some weighted boots to wear when outside. Without a tether, he'll just keep floating up. Krick needs more formula ingredients to make the effect permanent. Vince wants to help.
The compound from Koenig's blood is osmium mixed with lutetium - another very dense element. Also rare, it's mainly found in meteorites, so the team heads to the science museum. Vince has already entered through the skylight and let Krick in. The team catches them, and Krick bolts, but Olivia nabs him. Vince unstraps his harness and floats toward the open skylight, untethered. Peter tackles him before he drifts away.
Michael visits his dad in jail. Krick explains he hurt some people, but he wanted to fix them. They were in wheelchairs, like Michael. The boy leaves, crushed that his dad sees him as something that needs fixing. Krick tells Walter he's in aeronautics. A few months ago, he was working on a new alloy for military aircraft but not having much success - until he combined those heavy elements. Somehow, the result was a new, lighter-than-air molecule.
Peter confesses to Olivia that he's working on something but can't trust the FBI, or the overprotective Walter's judgment. However, he does trust Olivia's. So ... full disclosure.
In Nina's office, Walter says Krick wasn't changing the laws of nature; they're changing on their own. What's happening on the Other Side is starting here. But he knows how to stop it: soul magnets. Nina says, "Not this again." Walter picks up the bell William left to Nina. He insists that the harmonic vibrations from it will draw Bell's consciousness from whatever vessel he chose before death. Walter rings the bell, peers at Nina, and asks, "Belly?" Nope, it's still Nina. What now?
Peter shows Olivia his research. It's everything they know about the machine. She sees the memory chips and realizes how he got them. She turns away, and a chime echoes through her mind. Peter says he can't break the encryption. In Bell's voice, Olivia says, "That's because the decoder key is in my office." At the FBI? No, at Massive Dynamic. Olivia/Bell turns. "Hello, Peter. It's nice to see you again."