Supergirl, The Flash, Arrow and Legends of Tomorrow are all in a crossover series event where a new story gets unfold throughout a four-night, four-episode crossover of the four series.
This crossover event will take place when Flash and Cisco seeks Supergirl’s help with an alien invasion on their Earth, which will take place at the end of the Supergirl’s episode titled “Medusa”, which is set to air on Monday, November 28.
The next night, Tuesday, November 29, Supergirl will appear on The Flash’s episode, titled “Invasion“.
The Dominators (aliens) attack Central City. The Flash speeds over to Star City to ask Arrow for help in stopping the Dominators. They realize they can’t stop the Dominators on their own, so they track down the Legends of Tomorrow, and Flash and Cisco creates a breach to bring in Supergirl. Once all the superheroes are assembled, they come up with a plan to attack the Dominators, but a shocking secret related to Flashpoint is revealed and turmoil stirs up.
The following night, Wednesday, November 30, the story continues at Arrow’s episode, also titled “Invasion“.
In Arrow, Oliver (Green Arrow) wakes up to a life where he never got on The Queen’s Gambit. Robert and Moira Queen are alive and well. Laurel is his loving fiancée and their wedding is about to happen. Everything seems perfect, but Oliver starts to notice small imperfections that make him question this new reality. Meanwhile, Felicity and the recruits take on a new threat with help from The Flash and Supergirl.
The “Invasion” chronicle end on “DC’s Legends of Tomorrow”, Thursday, December 1, also titled “Invasion“.
After they learn the Dominators’ plan for the world, the Legends must work together with The Flash, Supergirl and Green Arrow to kill them once and for all. Stein figures out, with the help of others, how the team can terminate the Dominators, but is distracted by the anomaly he realizes he created in 1987.
To follow what happens during this time, you need to watch all four episodes. This means; watching Supergirl, The Flash, Arrow and Legends of Tomorrow on all four consecutive days, starting Monday November 28.