After Vandal Savage's attack on Central and Star City; Barry, Oliver, Cisco and Felicity came up with a plan that if one day a bigger threat was to occur, then they would activate code: STORMWATCH, an initiative to reunite known heroes to help save the world.
After defeating Damian Darhk, Non, Vandal Savage and Zoom, a new threat from space has appeared in Central City to take control of Earth. Now Teams Flash, Arrow, Supergirl and the Legends of Tomorrow must team up to save the Earth with the help of allies.
It has not been long since Barry Allen became The Flash and he has been doing his best to protect Central City. Supergirl has also not been active very long helping out in Metropolis where she can. They have both been working separately but know of each other's existence. However a new evil has emerged it's head, targeting other cities. Will Supergirl and The Flash be able to work together to overcome this threat.
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