Nate, Mick, Amaya, Felicity and Cisco travel back to the first Dominator invasion of Earth in Oregon, 1951. The Legends successfully capture a Dominator for information, but are themselves captured by Federal agents. While in custody, they learn from the Dominator that the aliens arrived to assess the threat humanity posed now that metahumans have appeared. Felicity and Cisco rescue the Legends and help set the Dominator free. Meanwhile in Central City 2016, the team learns that the Dominators know about Barry's manipulation of the timeline, and that they demand his surrender in exchange for peace. After the Legends return, the team discovers that the Dominators' weapon is a bomb that will kill all metahumans on Earth, with millions of collateral human casualties. The teams dissuade Barry from surrendering, with Cisco reconciling with him. They manage to destroy the bomb and force the Dominators to retreat using pain-inflicting nanotechnology. Cisco gives Supergirl a device to travel between their universes. Martin convinces Jefferson not to tell the others that his daughter, Lily, is an aberration of the timeline. This episode concludes a crossover that began on The Flash season 3 episode 8 and continued on Arrow season 5 episode 8.