<Tr> <Td_colspan="2"> List of Legends of Tomorrow episodes </Td> </Tr> <Tr> <Th> Preceded by </Th> <Td> "Heroes Join Forces" </Td> </Tr> <P> "Invasion!" is the third annual Arrowverse crossover event, crossing over episodes of the Arrowverse television series The Flash, Arrow and Legends of Tomorrow on The CW, and is inspired by the 1989 comic miniseries Invasion! . Events at the end of the Supergirl episode "Medusa" on November 28, 2016 marked the beginning of the crossover, which officially began on November 29 with The Flash, continued on Arrow on November 30, and concluded on Legends of Tomorrow on December 1 . Each of the latter three episodes were titled "Invasion!". "Invasion!" sees Barry Allen recruit Kara Danvers / Supergirl to his Earth to help his team, Oliver Queen and his team, and the Legends to defeat the alien race known as the Dominators . </P> <P> Development for a crossover between the three series, along with Supergirl, began in May 2016 after it was revealed that Supergirl would move to The CW from CBS for its second season . By the following September, the scripts for each episode had been written, with filming occurring shortly after, in October 2016 . The premise and episode titles were also revealed that month . "Invasion!" sees all of the main cast members for each series appear in at least their own series, with Melissa Benoist appearing as Supergirl across the entire crossover . Multiple actors and characters from previous Arrow seasons also return to reprise their roles, as Arrow's episode doubled as its 100th . </P>

What episode of legends of tomorrow is the crossover