Cryogenically frozen heads are loaded into a van at a research facility. A man (Roger Cross
) pulls up asking the two workers for directions. Then he shoots one. The other helps the man steal the van. As the thief drives away the other man says "let me know if he's in there, you know where to reach me."
One of the lab workers shoots the inside man. He fights, but finally goes down. The body bleeds silver.
In the lab, Walter (John Noble
) tells Olivia (Anna Torv
) he has a thought about how she can retrieve her memories from the alternate universe. He wants her to eat black worms. He remembered an experiment he and William Bell (Leonard Nimoy) did in which worms exposed to light remembered it and transferred that light to other worms. Astrid (Jasika Nicole
) has blended up a worm smoothie for Olivia. As Peter (Joshua Jackson
) and Walter argue over the theory's validity, Olivia grabs the worm puree and drinks it down.
They get a call to head to Elias Cryonics in Medford.
Broyles (Lance Reddick
) tells them about the robbery - it's the third robbery of its kind in a week at a cryogenics facility.
Walter gets to work on the silver blood, which looks like mercury.
As Peter walks toward Olivia she has several alternate universe flashes, including a glimpse of William Bell and the towers.
Shapeshifter Charlie Francis (Kirk Acevedo
) shows up. He sees Walter find a device like the one he used on the dead body.
The thief examines his frozen head bounty at a remote location. Charlie joins him, not looking so hot. He tells the thief he was sloppy last night, leaving one of their own dead for the FBI to examine. The head hunter tells Not Charlie he needs to get home and get fitted with a new conversion device, he's dying in Charlie's body.
The thief finishes looking at the heads, but doesn't find the one he's looking for.
Charlie suggests Olivia might know where the head is hidden. He thinks that's why Bell brought Olivia over.
Olivia tells Broyles they think shape-shifters from another dimension are looking for a specific head, but they don't know which one. Olivia wants Peter to look at the conversion device.
Broyles thinks Olivia might need a protective detail because the last time they found a device like that, she was attacked. She replies if they wanted her dead, she would be.
Walter examines the dead body, which is not human. He thinks the mercury acts as a highly evolved machine. He suddenly wants to call Olivia right away.
Peter examines the conversion machine as Olivia watches. He hooks it to something and turns it on, it's already streaming data.
Walter tells Olivia the shape-shifter's blood is half mercury, but the blood of the first one wasn't. Which means the first shape shifter wasn't the woman they killed, it's still alive. But Walter thinks he might know how to find it.
Olivia reports Walter's hope to Not Charlie, who is standing in line at a pharmacy to buy a boatload of thermometers.
In his car he breaks them open and drinks the mercury. General twitching and writhing follows. Then he calms down.
Walking down a hallway in Massive Dynamic, Olivia has a few more alternate universe flashes. She shows Nina (Blair Brown
) the conversion device. The MD scientists think each device is tuned to a specific user, which means because the shifter who tried to kill her no longer has his, he's trapped in the body. They're trying to figure out who it is, too.
Nina gives it to a super geeked out scientist, Kurt (Aaron Craven
) who says he should have images from the intact device in three hours and stream the images on a secure site.
The Bishops visit Walter's former experiment subject. He's nervous to see her, but Rebecca (Theresa Russell
) recognizes him immediately and hugs him. She tells them she saw a glow around shape-shifters. She calls it a great gift, but says the ability comes and goes. She agrees to do the experiment again - which involves taking a ton of LSD.
Back at the lab, they hook her up. She remembers how to set everything up. Olivia has Astrid log onto the MD site and link it to her phone, too.
Walter injects Rebecca with the first round of psychedelics. Olivia watches as the first basic image comes across from MD. It's just an outline of a head at this point.
Rebecca tells Peter she met him as a baby. She says "I swear I saw..." but the drugs kick in. Walter tries to guide her through seeing the shape-shifters, keeping her calm. He describes the ringing of a bell on campus, and Peter rings one. Upon hearing the noise, Olivia crumples to the ground.
Suddenly, she's in a room with William Bell, who's telling her it's nice to see her after all these years. She's remembering her visit.
He apologizes for the way he brought her there. He tells her to call him William, or Willem, which is what she called him as a girl. Time fuzzes a little for her, and he says she's having problems with time slipping, but she's lucky because someone without her natural talent would have more problems with it.
He asks what Walter has told her. She mentions the drug trials they did on her and says the others are beyond permanently damaged.
But he tells her she's just coming in to her ability.
He says a war is coming, as he and Walter predicted. They knew they needed a guardian, someone to watch the gate between this side and hers. He says he can't go back yet and only a few people have done it safely.
On his side, they've designed hybrids, part humans, part machines that are part of the "first wave" on her side. She tells him she doesn't trust him and thinks he started the war.
He tells her he's more of an ally than she thinks. He warns her a storm is coming and he thinks when it's over there will be little left of his adopted world. The shapeshifters are looking for someone who can open the door, he tells her she has to find him first - she was the strongest of all the children they prepared.
He draws a symbol and tells her she'll find it on their leader. It looks like 80 percent of a circle with x's marking the open ends.
He rings the bell and tells her to stand, telling her it'll be less painful that way.
In her world, Walter explains to Peter he thinks she's having a flood of memories that are overpowering her. Bell tells her the Greek phrase we heard her say in the season premiere to Peter.
He warns her that he pulled her out of a moving car and momentum must be repaid. She remembers going through the windshield as Peter stabs her with adrenaline and she wakes up like a shot, saying she needs to talk to Nina.
Olivia calls Nina, telling her she has a message from William Bell.
On campus, the Bishops say good-bye to Rebecca. Except Walter wants to go home with her. Peter says OK and gives his dad some money. Peter walks over to say good-bye to Rebecca. She starts for a moment as she sees a glow of light around him, but it could just be sunlight. She dismisses it as just the drugs.
Not Charlie comes to the mostly empty lab and finds only Astrid there. He's looking for Olivia. He sees the MD shapeshifter recreation on the computer screen, looking more and more like him.
Nina tells Olivia she's never seen the circle symbol before. Olivia tells her about the storm coming and the man being able to open the door between the worlds. Nina says Bell always worried about the worlds colliding. She demonstrates the principle by bashing together two snow globes. Only one remains in tact.
Olivia remembers the name of a cryogenics lab Bell told her about. Her phone buzzes. It's a text from Charlie telling her Nina is the shape-shifter.
Olivia excuses herself.
Charlie is waiting for Olivia outside. He tells her SWAT is about to storm the place. She tells him she almost told Nina everything - except the name of the cryonics place. She tells him. She takes out her phone to call Broyles, but it beeps at her, the MD rendering is complete. Charlie's face is staring back at her.
Not Charlie is staring her down on the street.
Charlie throws her to the ground. She tries to fight, but he knocks her down and kicks her. He calls someone, telling them the lab's name. Olivia gets her gun out, but he fends her off. He gets her on the ground, choking her. She grabs wildly for her gun nearby. A passerby sees Charlie attacking her and shouts for him to stop. Not Charlie shoots him dead. But it gives time for Olivia to get her gun on him.
She hesitates, unable to kill her friend. But he draws on her. She shoots him between the eyes.
Rebecca takes Walter home, but he declines her invitation to come in. He tells her what he did was wrong, taking advantage. She tells him no and kisses him. She says he made her special. He says she always was and leaves.
Broyles consoles Olivia, saying she had no choice. And it wasn't Charlie anymore. She doesn't understand who the people are who are doing this. Broyles tells her the bad news: the lab was hit 15 minutes before their team got there. It was cleared out.
She thinks she failed.
Somewhere, a man's frozen head is unwrapped. It bears the symbol. It's attached to a body and animated with mercury. Its eyes open.