Pete and Myka go to St. Louis to investigate an alleged gas main leak that caused an explosion in the basement of a police station. Going in past the emergency generators, they check the body of one officer, burned to a crisp, and meet with Captain Powell. He’s not happy to have the Secret Service there. The agents check out the basement and find a cellar that was built over when the current police station was constructed. Inside they find a long-dead corpse, manacled to the wall. They go through his clothing and find a wallet identifying him as a Secret Service agent. They also find a Tesla gun.
The agents contact Artie and tell him what they’ve found. The corpses ID papers have rotted and Artie admits they’ve had a lot of agents over the year. While Artie checks the agent records, Claudia uses a device she’s newly built to project holograms of file photos. She brings up a hologram of the corpse and starts reconstructing the missing parts.
At the police station, Pete and Myka go over the security tapes and determine that one prisoner, Reggie Hinton, has disappeared in the confusion. Powell puts out an alert. Meanwhile, Reggie has gone to where his gang hangs out. He warns them that someone is after them. As they run, someone arrives from the shadows, radiating electricity.
Pete, Myka, and Powell get word about the fact the gang was killed. They arrive there and discover that the gangers were charred to death. However, one of them has an unburned hand mark on his skin. At the warehouse, Artie has spotted a partial tattoo on the cellar corpse’s arm. Claudia manages to reconstruct enough of it to identify it as a Marine tattoo, helping Artie narrow down his search of the agent files. Myka and Pete check in and they figure that the deaths are deliberate. Either the artifact caused the explosion at the station or the explosion released the artifact. Either way, Artie suspects that the deaths are deliberate. The agents ask Powell to talk to the head of the gang unit, Clark, but Powell informs them that Clark is on mandatory leave.
Artie and Claudia get the agent’s corpse and use Claudia’s new scanner to reconstruct its face. Artie checks the face against the agent photos of ex-Marines and identifies the man as Jack Secord, recruited in 1955 and last seen in Minneapolis. Artie goes to a special file room that “files” MIA agents’ rooms from the bed & breakfast and brings up Jack’s room there. He and Claudia go through the room and find a note he wrote to a woman he was meeting. Using Jack’s writing Pad, Artie learns that the note was addressed to a Rebecca Sinclair living in St. Louis.
The agents go to see the now elderly Rebecca. She admits she knew Jack and seems to know a lot more than she’s admitting. Pete and Myka discuss the case in general terms with her as she brings them cookies. Myka gets a call about another charred corpse and they prepare to leave. As they go, Rebecca wonders if the new victim was also electrocuted.
At the crime scene, there are more dead gang members. The one surviving teenager says that a cop killed his friends. The agents figure Clark is involved and Powell confirms that Clark is the only officer unaccounted for. Pete and Myka go to Clark’s address and break in when they see electrical sparks. Clark is clutching at a lobster-like device on his back that is releasing the electricity. He jumps out the three-story window and survives the fall, unharmed. He runs off and the agents follow only to discover his body lying in a nearby alley. The artifact has detached itself and fled.
Myka and Pete send a photo of Clark’s corpse to the warehouse and Artie and Claudia confirm the marks on his back match the ones on Jack’s corpse. Myka remembers that Rebecca mentioned electricity as the cause of death when they hadn’t told her and go back to see her. Rebecca admits that she knew Jack was an agent and that she was also a warehouse agent.
Artie locates the artifact in a book and tries to translate the entry. In St. Louis, Rebecca shows Pete and Myka photos of similar electrical deaths that took place just before Jack’s disappearance. The murders started occurring at the same time that an exhibit of 11th century artifacts came to the city museum. Rebecca says that Jack was known for his intuitions and followed a hunch on his own, and she never saw him again. They were in love but Jack’s first love was seemingly to Warehouse 13. When Jack disappeared, Rebecca changed her name and dropped out of sight. Going through Jack’s files, Pete finds a reference to the “Hand of God.” Artie confirms that the artifact is the Spine of the Saracen, used during the Crusades in the 11th century to give a warrior superhuman strength. They figure the artifact was part of the museum display and was somehow activated. According to the texts, the artifact requires a lifetime commitment from its host.
The agents go back to check Clark’s apartment and hear a gunshot. A woman runs out of a nearby building and tells them she shot her ex-husband. The man comes out of them, still bleeding from the chest, and the agents use the Tesla gun without affect. The man finally dies but the artifact crawls into the bushes before the agents can stop it. They get the woman way and try to find the Spine of Saracen. It leaps out of the bushes for Myka and Pete throws himself in the way. It attaches itself to his back and Myka tries to pull it free. It shocks her unconscious and Pete runs off, screaming in agony.
A few minutes later, Powell finds Myka and revives her. She calls Artie, who has learned that the Spine turns the bearer into a killing machine using his own life to power him up. When the person uses up all of their energy, they die. Rebecca arrives as Myka concludes that the host is driven to attack whomever or whatever they hate the most. Jack locked himself in the cellar rather than endanger anyone, and Myka figures Pete will do the same. When Claudia suggests that electricity might be able to stop the artifact, Myka figures that Pete will come to the same conclusion and use the emergency generators at the police station to destroy the Spine of Saracen.
At the police station, Pete is trying to hook the power cables up to the Spine. Myka arrives and tries to convince Pete they can find a cure but he insists he can’t fight the Spine’s influence much longer. He asks Myka to help him attach the cables. She refuses, but Rebecca arrives and does so for her. As Rebecca prepares to throw the power switch, Myka says that she knows Pete will come back. The generators feed electricity into the artifact, weakening it and knocking it loose. As Myka administers CPR to Pete, Rebecca crushes the Spine with a fire extinguisher.
Later at the warehouse, Pete and Myka bring Rebecca back and introduce her to Artie. She hands over the remains of the Spine and says that now she’s completed Jack’s mission, she’s done. Artie offers her a position but Rebecca isn’t interested. Artie gives her a last chance to see Jack’s bedroom. In it is the engagement ring that Jack was going to give her: he was going to choose her over the warehouse after all. As Myka stands there with her, Rebecca tells her to get out while she still has a life to live, and before Warehouse 13 uses her up entirely.