A Nazi regime rules the parallel world of Earth-X. On Earth-1, Barry Allen and Iris West's friends, including Kara Danvers and Alex Danvers from Earth-38, come to Central City for Barry and Iris's wedding. Oliver Queen re-proposes to Felicity Smoak. The wedding ceremony is interrupted by invaders from Earth-X led by Dark Arrow, a Kryptonian called Overgirl, and the Earth-X analog of Prometheus. 

In S.T.A.R. Labs, a captured inter-dimensional villain from Earth-X, Prometheus, is revealed to be a parallel universe doppelgänger of Tommy Merlyn. After he cruelly taunts Oliver over his attachment to Earth-1's Tommy. Prometheus's partners, Dark Arrow, Overgirl, and Eobard Thawne steal a sublight generator, the Prism, from research company Dayton Optical Systems; Oliver, Barry Allen, Kara, and the others believe that Dark Arrow seeks to build a neutron bomb with it to aid in his conquest of Earth-1. Oliver, Barry, and his allies are abducted to Earth-X's concentration camp, but Kara is moved to S.T.A.R. Labs. 

In the concentration camp, the heroes are rescued from execution at the hands of SS-Sturmbannführer Quentin Lance by Ray Terrill and Leo Snart, Leonard Snart's doppelgänger. Thawne prepares to operate on both Overgirl and Kara, and Iris and Felicity work to rescue their friends at S.T.A.R. Labs. General Winn Schott, the commander of the Freedom Fighters, is determined to strand Dark Arrow and Overgirl on Earth-1 by destroying the temporal gateway. While posing as Dark Arrow, Oliver discovers that the Nazis possess a doomsday device in the form of a timeship called Wellenreiter, a militarized equivalent of the Legends' Waverider. 

The heroes return to Earth-1, and Iris, Felicity, Kara, and the others are rescued by the returned heroes and the Waverider crew. Martin uses the serum to separate the Firestorm matrix, dying from his wounds. Harry, at the helm of the Waverider, destroys the Wellenreiter after the heroes disable its shield. During battle with Kara, Overgirl's solar radiation goes nuclear.