In the wreckage of the Warehouse, which has been destroyed by Sykes's bomb, Artie clutches a pocket watch as though it may save them yet. Upon noticing that the strongest chamber of the Warehouse was incinerated, Artie runs to Leena's for information. Myka and Pete follow in confusion, which amplifies when they learn of Mrs. Frederic's death. Upon watching the news, Artie sees that chaos is spreading over the globe - Pandora's Box was in that super-strong chamber, and when it was decimated, so was hope. Only the watch Artie holds may be able to get it back.
When he presses a button on the watch, it turns into a stopwatch with 23 hours in its countdown. That means they have almost a day's grace in which to find a time travel device and mend this catastrophe. Clues found on the watch lead them to a Knights Templar stronghold in France, and Leena hangs back with a Warehouse insider to find an artifact that can neutralize Sykes's bomb, in case the others can turn time backwards.
Their destination is heavily guarded by black-robed Knights Templar, so Myka and Pete "distract" (read: fight) the guards while Artie and Claudia find what they came for: Magellan's astrolabe, a navigation tool. Just as they notice it's missing a piece, the head of the sect finds them and attacks. Pete and Myka get there in time to Tesla him, but he's trapped Claudia in a tunnel. The watch turns into a time-teller again and shows them the number "4:18". Together, they work out that it refers to a chapter and verse from the Bible that references St. Peter. With time running out on the fate of humanity, they have to leave Claudia behind and race to St. Peter's Basilica in The Vatican.
In Rome, Artie knows of a restaurant with a secret entrance to the Basilica. Myka throws a chair through the restaurant window, distracting the restaurant staff long enough, but the police arrest her. Artie and Pete must press on without her.
Under the Roman streets, they find an elevator shaft with the astrolabe's missing piece, but the head of the Knights Templar arrives and attacks again. Pete manages to fell the man, but is fatally wounded in the process. Artie says goodbye to Pete and realizes that if the astrolabe can't turn back time, he is all alone. As he fits the pieces together, the head of the Knights revives and warns Artie that if he uses the astrolabe, he will create an evil that will follow him for the rest of his days. With no other option, Artie assembles the astrolabe and watches the world spin in reverse around him.
He is back in the Warehouse, moments before the explosion. He, Myka, and H.G. rush to find Ghandi's dhoti, an artifact the pacifism of which could counteract the bomb, and apply it, but then realize that it's not the bomb that needs counteracting - it's Sykes. They capture Sykes and wrap him in the dhoti, and watch as the evil leaves his body, and the bomb stills and silences.
Later, Artie campaigns for amnesty for H.G., and though Mrs. Frederic senses something amiss, she agrees to follow his judgment. Exhausted, Artie sleeps at his desk and dreams of Claudia chasing him down and stabbing him. He wakes to a message that Claudia is gone - she slipped away in the middle of the night.