Last episode, the team found out Paracelsus was the former Caretaker of Warehouse 9 and he’s now trying to take over Warehouse 13. Since there can be only one Caretaker, Mrs. Frederick (the current one) is slowly losing her mind as Paracelsus takes over. Artie tells everyone that Paracelsus has to be taken alive and Bronzed, as killing him would shatter Mrs. Frederick’s mind completely. Claudia asks about Artie taking blood from Joshua to stabilize her and Steve’s lie detection sense tells him Artie didn’t get the blood from Joshua. Artie orders everyone to work on finding Paracelsus and figuring out a way to stop him taking over. Myka makes an appointment with her oncologist, which makes Pete happy since he’s the one who basically browbeat her into doing it.
Artie checks on Mrs. Frederick, who’s urging Abigail to find a safe place to hide away from the Warehouse. We found out last episode the Abigail is the Keeper, the living repository of the Warehouse’s knowledge. Abigail doesn’t like running out on her friends, but finally agrees to go somewhere safe. Claudia gets reports of Paracelsus from three different hospitals in San Francisco. Paracelsus was carrying a glowing golden bowl into the cancer wards at each hospital, giving numerous patients a whiff of the smoke, but none of the patients have died. Pete says he’ll check it out alone, but Myka talks him into letting her come along, since her doctor’s appointment isn’t for a few days.
Sutton and Nick apparently returned to the Warehouse with the others and Sutton tries to talk to Nick. But Nick’s pissed off that his father lied about Paracelsus’s immortality formula (Sutton forgot to mention Paracelsus powered it by killing 600 innocent villagers); Nick says it’s too late for Sutton to act like a father now, and since his mother is dead, he’s completely alone. Nick may be mortal now, but he’s definitely still a brat. In San Francisco, Pete and Myka check in the terminal ward at one of the hospitals, asking about Paracelsus. Pete is a little freaked out being surrounded by so many dying people, and can’t stop thinking about Myka’s cancer. The doctor says Paracelsus had a glowing bowl with him and all the patients who breathed in the smoke seem to be in complete remission … Paracelsus cured them.
Pete wonders if Paracelsus is trying to help people (and you can tell he hopes Myka’s cancer might have a chance to be cured), but Myka reminds him of the villagers Paracelsus killed 500 years ago, not to mention Charlotte last episode. They talk to a woman named Maggie, who Paracelsus cured; she says he told her he could see the cancer in her eyes and was going to take away her pain. He said his purpose would be clear once he reached the place “where air has yielded to stone”, then he moved on to another patient. Pete and Myka figure Paracelsus is using the Philosopher’s Stone, but can’t figure out what his motive is. Claudia calls to say Paracelsus was seen at another San Francisco hospital.
Artie comes in with the special ribbon that’s used to transfer power between Caretakers. Claudia freaks, thinking he wants to transfer Mrs. Frederick’s Caretakership to her (Claudia was earmarked as the next Caretaker a while back), but Artie tells her he has something else in mind. He’s hoping the ribbon can sever Paracelsus’s connection to the Warehouse remotely, by sending energy through the Farnsworth communicators. Pete and Myka check the hospital where Paracelsus was seen and the security chief says their surveillance doesn’t show him leaving, so he might still be there. Pete and Myka head to the morgue (where Paracelsus was last seen) and hear his voice from the back of the room. He tries to assure them everything he’s doing is in the name of progress, but when they track down his voice, it’s coming from an artifact. Pete’s tired of Paracelsus’s crap, but Paracelsus sends a shock through the artifact and says he won’t hesitate to kill Pete or Myka if they get in his way.
At the Warehouse, Claudia gives Nick shit for being all emo about his dad. She reminds Nick that he almost killed her, but she forgave him because he was just trying to do the best he could in a bad situation; she suggests Nick extend Sutton the same understanding. She asks Nick about the immortalization and he mentions that a wall of the dungeon was covered in wood … petrified wood. Claudia tells Artie and they call Pete and Myka to fill them in. Petrified wood is formed when oxygen is driven out of plant cells and replaced by minerals, so it fits the “air yielding to stone” stuff Paracelsus was talking about. Artie says Paracelsus used a very specific type of petrified wood, only found in an area of Eastern Turkey. That area is now developed, but some of the wood was sent to San Francisco, to Lillie Hitchcock Coit, the benefactor of Coit Tower. Artie says the Tower has a secret inner wall made of petrified wood.
Artie shows Pete and Myka a cog once belonging to John Logie Baird (TV pioneer) with the Caretaker ribbon wrapped around it. He’s hoping they can Tesla the beribboned cog, send the energy through the Farnsworth, and blast Paracelsus, disconnecting him from the Warehouse. Artie hopes it works because Mrs. Frederick is fading fast and they might not have enough time to actually bring Paracelsus back to the Warehouse. At Coit Tower, Paracelsus has broken through the wall and is preparing the Philosopher’s Stone when Pete and Myka show up. Sutton pops up and tries to shoot Paracelsus; he misses, but it screws up Myka’s surprise. Paracelsus uses Genghis Khan’s Mace to redirect Myka’s Tesla shots, first at Sutton then back at her. Pete uses the Farnsworth to direct Claudia’s Tesla shot through the ribboned cog (from back at the Warehouse) at Paracelsus. But Paracelsus uses the Mace to toss the energy right back at Pete, knocking him out and giving Artie some painful feedback.
Paracelsus uses the Stone to give himself immortality, which causes all the hospital patients he “cured” to simultaneously flatline, while back at the Warehouse, Mrs. Frederick keels over. Paracelsus says progress demands sacrifice, which makes Pete shoot him. But he’s immortal now, so he just laughs it off and runs, with Pete right on his ass. Pete chases Paracelsus toward the wharf and calls Claudia to tell her he has an idea, which worries Artie. (Claudia: “He’s a smart boy when he’s not thinking about cookies or boobies.”) Pete has Claudia hack into the drawbridge controls, cutting off Paracelsus’s escape route and giving Pete a chance to grab him.
They take him back to the warehouse to be Bronzed, but it doesn’t work. Since Paracelsus is the one who invented Bronzing, it makes sense he knows which chemicals to ingest to prevent it. Pete suggests chopping his immortal ass into tiny pieces, but they can’t kill him before severing his link to the Warehouse if they want to save Mrs. Frederick. Artie explains the ribbon isn’t strong enough to overcome Paracelsus’s will, so he says they’ll put him in the suspension chamber until the Regents show up. Meanwhile, Nick gives Sutton shit for trying to play hero and almost getting killed. Sutton says he was trying to make up for all his past mistakes and tells Nick he’ll never be alone. They leave together after Pete and Myka hastily arrange new identities.
Myka tells Pete her doctor is worried about some pain she’s been having and wants to do some surgery today to biopsy the mass and see if it’s spreading. Pete says he can go with her, but she’d rather go alone. Pete has a thing about losing people, so he’s pretty freaked out, but Myka says she isn’t going anywhere. As she leaves, it’s obvious she’s just as scared as Pete is, but she’s trying to put a brave face on. Inside the Warehouse, Mrs. Frederick is getting worse, so Pete decides to interrogate Paracelsus a little less politely than usual. Steve asks Artie about Joshua’s supposed blood sample (since he found out Joshua’s been in Switzerland) and Artie tries to evade the question, saying the DNA he used is from Claudia’s family but it’s better if she doesn’t know about it. Mrs. Frederick wakes up long enough to urge Artie to tell the truth, saying Claudia will find out anyway if she becomes the next Caretaker.
Pete tries to get info from Paracelsus but Paracelsus turns the conversation toward, Myka. He says he could see her cancer and it’s spread too far for any surgery to help. He urges Pete to use some artifacts to save her, even offering to tell him which ones will work best. It’s obvious Paracelsus is manipulating Pete, but he’s so worried about Myka he can’t think straight. Paracelsus tells Pete about a couple of artifacts that can cure Myka with minimal side effects and asks for a favour in return. Paracelsus knows the Regents will lock him away forever and he says his scientific mind can’t handle that, so he begs Pete to ask the Regents to erase his mind and memories. Pete knows that’s possible, since the Regents did that to H.G. Wells.
Artie tells Claudia the truth about the DNA he used to stabilize her … it came from Claudia’s sister, Claire. One problem with that—Claire supposedly died in the same car crash that killed Claudia’s parents. Artie says Claire didn’t die in the accident, she’s still alive. Claudia’s pissed off that he kept it from her (though apparently Joshua knew), and her mood doesn’t improve when she learns Artie has known all along, since before he even met her. Artie tells her he was trying to protect her because Claire is incredibly dangerous. Pete gathers the artifacts to save Myka but Paracelsus gives him such complex instructions he can’t possibly follow them. Pete figures he can free Paracelsus and let him do it, which is probably just what Paracelsus was hoping for. But Pete’s not quite that desperate; he says first he’ll make Paracelsus mortal again (thus assuaging his fears about immortal punishments) and then hold a gun to his head while he comes up with something to save Myka. Paracelsus isn’t thrilled about that idea, but Pete says it’s that or nothing.
Pete uses the Stone to remove Paracelsus’s immortality, but it doesn’t work out the way he planned. Turns out using the Stone inside the Warehouse (close to the Eldunari, the Warehouse’s heart) increases the Caretaker’s power exponentially; I guess Paracelsus forgot to mention that. He now has complete control of the Warehouse and everything in it, which he demonstrates by freeing himself from suspension and wrapping Pete up with a rope. Artie and Claudia’s argument is interrupted by a strong tremor running through the Warehouse. Claudia can sense Paracelsus’s power and quickly severs Mrs. Frederick’s connection to the Warehouse, saving her life. Mrs. Frederick says Paracelsus is in complete control and they have to leave.
Luckily, Paracelsus isn’t in a killing mood, he just wants everyone to get out of his Warehouse, so he deposits Pete with the others. He encourages them to leave with lightning bolts and falling shelves, herding them toward the back exit. As they’re about to leave, Claudia stops and says the Warehouse needs her to stay and fight Paracelsus … she can feel it. The others aren’t happy, but Claudia says the Warehouse will protect her, allowing her to fight Paracelsus from inside while they fight from outside. They reluctantly agree and leave her behind; as soon as they get outside, Paracelsus throws a force field over the entire Warehouse, blocking them out and Claudia in. Pete figures it’s his fault because of the artifacts he gathered for Paracelsus earlier. Meanwhile at the hospital, Myka’s surgery is over but she hasn’t woken up yet. Since this is the last episode of Season 4, we’ll have to wait months to see what happens next … oh no, we’ll only have to wait a week.