Pete, Myka, and Artie survey the remains of Warehouse 13 after Sykes’ artifact nuclear bomb detonated. The agents ask Artie what he meant about McPherson’s pocket watch and ask if it can rebuild the warehouse. He tells them that he doesn’t know what the pocket watch does, but believes that it is a key to what might be a fail-safe device. Artie hesitates and says that he feels something, and then runs to what used to be the Ytterbium Chamber. He explains that it was supposed to be impenetrable and he had hoped it would protect what was inside. Artie then runs off to Leena’s to check the television.
When the trio arrives at Leena’s, Artie warns them that he doesn’t know what the fail-safe does or what the downside is. They find Leena and Claudia, mourning, and Artie explains that when the warehouse was destroyed, Mrs. Frederic died as well because she was organically bound to it. Leena and Claudia are shocked to learn that the warehouse is gone, but Artie tells them to focus. He explains that Helena sacrificed her life to save them when Sykes’ bomb exploded. Pete tells Artie to use the watch, saying it doesn’t make a difference, but Myka warns that it could be dangerous. Claudia wants to use it immediately, but Myka warns her that they need to be cautious. Leena asks about the Ytterbium Chamber and Artie explains that Pandora’s Box was destroyed. He turns on the TV and they listen to news reports of the financial markets crashing across the globe. Worse, people are rioting and committing suicide in huge numbers due to extreme depression. Artie tells them that with the destruction of Pandora’s Box, hope has been lost.
The team examines the watch and finds the name Barbosa inscribed on the dial. The words “Carregar O Botao” is written on the back, and Myka translates it as meaning “Push the button.” Artie does so and the watch becomes a stopwatch showing 23 hours remaining even though it has 24 hours marked off. They figure that it started counting down from the moment that the warehouse was destroyed., The back panel is loose and they open it to discover that it was made with black diamonds rather than rubies.
Artie gets an idea and heads back to the warehouse. He triggers a remote device as the others arrive, and Pete’s football drops out of the sky. Pete catches it and Artie opens it to reveal an elaborate computing device. He explains that it’s an artifact tracker that downloads information to the warehouse system as it passes overhead. It has an entry on the Knights Templar relating to black diamonds and a location in France at Richerenches. Artie says that the Templars had an underground sect there and Claudia checks the Internet and tells them that the Knights of the Black Dragon were located there. Artie has Leena talk to Regent Keeler, a history expert, and find an artifact that can undo the effects of the bomb once they rewind time. As they head for Richerenches, Claudia says that she’s staying because whatever they do won’t bring back Steve. She plans to use the artifact metronome to raise Steve from the dead, but Pete points out that he’d be coming back to a world without hope. The agents say that they need her and Claudia reluctantly agrees. However, she warns Artie that she will remember what she has to do no matter what.
In Richerenches, Artie confirms that they have ten hours left. There are advetisements for black truffles, also known as black diamond, and Artie explains that the Brotherhood collects and auctions truffles for the church. The group feels cold and Artie explains that without hope, their minds are interpreting things in a duller context. He says that they can focus on other emotions to resist the loss of hope. They spot a sign for the converted Barbosa Commandry and connect it to the name on the watch. They hail a cab and prepare to leave for the commandry, and Pete points out that two men are following them. Artie figures that their members of the Brotherhood and they’re not being inconspicuous because they’re on their own turf.
Leena meets with Keeler and calls the team in the field. They’ve confirmed that the artifact bomb was made from a piece of masonry from London in World War II, filled with the Nazis’ hatred. They think an artifact of pure peace could neutralize it and go back to work.
At the commandry, the team sneaks in and sees brotherhood members unloading crates. Pete and Myka send Artie and Claudia on while they create a distracting pretending to be annoying American tourists. When that doesn’t work, Pete punches the biggest Templar and a fight breaks out while Artie and Claudia sneak in disguised as deliverymen. They find a cellar with wine casks and realize that they conceal a hidden door. They’re unable to shift them until Claudia finds a hidden panel that unlocks them and lets them shift. On the other side of the caskets is a boarded-over tunnel. They pull the boards away and Claudia goes in. She finds an astrolabe and hands it to Artie, who confirms that it belonged to the famous explorer Magellan, who was the first man to circumnavigate the planet. They figure that the M on the stopwatch stands for Magellan and that it relates to 24 hours. The alidade—the eyepiece—is missing and Claudia goes back in to find it. However, the leader of the Templars, Brother Adrian comes in with one of his men. He refuses to let Artie take the astrolabe and has his man advance on Artie. Artie stuns him with a Tesla and Brother Adrian throws a knife, activating a lever dropping a gate down and sealing Claudia inside of the tunnel.
Pete and Myka arrive and stun Brother Adrian, but they confirm there’s no way to release Claudia. Artie blames himself but Claudia tells them to find the missing alidade and rewind time so that she will never become trapped. They try to work out where the alidade is and check the watch, and they discover that it is now showing the wrong time of 4:18. They call Leena and Keeler, who have no luck with the numbers. Pete suggest that it might represent military time and Myka realizes that it refers to a Bible verse. The M refers to a book of the Bible and they confirm that it is from the Book of Matthew. The verse refers to the Rock of the Church, and they realize that it is directing them to the Vatican. The cornerstone was laid on April 18and they prepare to leave for the Vatican. The watch starts counting down again, revealing that they have only eight hours left. Artie hesitates to abandon Claudia and both Pete and Myka offer to stay. Claudia tells them to get the hell out of there and make the plan work.
In Vatican City, the police are trying to deal with rioters. The trio goes to a restaurant and Artie goes in. The owner, Bernadetta, refuses to let him into the basement and calls her burly cook down on them, and he hastily leaves. Outside, Artie explains that there is a secret escape tunnel leading to the Vatican. Realizing that they’re out of time, Myka shatters the restaurant window and runs off. The owner and her staff go after her, but she runs into the police and they capture her. She mouths for Pete and Artie to ignore her and get the alidade before it’s too late.
Pete and Artie make their way to St. Peter’s Tomb and try to find the elevator going down. With thirty minutes left, Pete starts to get major vibes, both good and bad. The elevator controls don’t work and Artie short-circuits them, and they notices that the floor dial inside the elevator has the alidade as a pointer. However, Brother Adrian and two of his men arrive and attack Pete. He disposes of the two Templars but Brother Adrian lunges at him with a knife, shoving him into the elevator. The doors close and when Artie opens them, he discovers that Brother Adrian is stabbed in the chest. Pete has the alidade but then collapses from the stab wound that Brother Adrian gave him. He collapses dying, and asks Artie that they got it. Artie shows him the alidade and Pete asks if it’s going to be okay because he won’t remember dying. After a moment, Artie says that Pete won’t... but he will.
Artie places the alidade on the astrolabe and then calls Leena and Keeler. They tell him that they’re best chance is to use Ghandi’s dhoti, which emanates an aura of pure peace. Artie signs off and prepares to use the astrolabe, but the dying Brother Adrian asks him why. Artie says that he can’t and Brother Adrian says that something irrevocable has happened. When Artie says he can fix it, Brother Adrian warns that anyone he tells afterward will be in grave danger. Artie agrees and Brother Adrian says that if he uses the astrolabe then he will create an evil of his own making that will live with him for the rest of his days.
Once Brother Adrian dies, Artie activates the astrolabe and rewinds time to the point where Myka and Helena were caught in Sykes’ rope trap. He hides the astrolabe as they free themselves and tells them that Sykes has an artifact nuclear bomb in his wheelchair. They wonder how Artie knows but he says that it doesn’t matter. They get the bomb from the wheelchair and go to the aisle where the dhoti is stored. However, the dhoti has no effect on the bomb. They realize that it is Sykes’ hatred that is keeping it going and that they have to use the dhoti on him. Artie remembers that Pete is preparing to trap Sykes in limbo and runs off.
Sykes and Pete struggle on the edge of the portal and Sykes goes through. As Pete prepares to shut down the portal and trap Sykes, Artie and the others arrive and Artie tells Pete to bring Sykes back. Pete goes through and brings their nemesis back, and Artie covers him with the dhoti. The bomb shuts down and they remove the dhoti. The image of the child Sykes once was passes over his face and he says that he’s sorry as he dies.
Later, Artie asks Mrs. Frederic and Adwin Kosan to trust him when he says that they can trust Helena. He insists that she would sacrifice her life for the warehouse, even though he can’t explain why. They don’t understand why he changed his opinion of Helena, but agree to pass his recommendation onto the remaining Regents and believe it will be sufficient. However, once Adwin leaves, Mrs. Frederic rounds on Artie and demands to know what really happened. He denies knowing anything further, but Mrs. Frederic reveals an odd streak scar on her head. When Artie refuses to confirm her suspicions, Mrs. Frederic disappears and he remembers what Brother Adrian warned him about. Artie dozes off and dreams of Claudia leaving Leena’s house and then chasing after him with a knife and killing him. When Artie wakes up, Leena calls to tell him that Claudia has disappeared.