"Invasion!" is the third Arrowverse crossover event, with episodes of the 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 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 main three episodes were titled "Invasion!". In the crossover, Barry Allen recruits Kara Danvers / Supergirl from Earth-38 to Earth-1 to help his team, Oliver Queen and his team, and the Legends to defeat the alien race known as the Dominators.
Development of a crossover between the three series and Supergirl began in May 2016, when it was learned that Supergirl would move to The CW from CBS for its second season. By the following September the scripts for each episode were written, and were filmed in October 2016. The premise and episode titles were also announced in September. In "Invasion!", the main cast members of each series appear in (at least) their own series; Melissa Benoist appears as Supergirl in the entire crossover. A number of actors and characters from previous Arrow seasons returned to reprise their roles, since Arrows episode doubled as its 100th.
The three episodes of the crossover received positive reviews, with critics praising the interaction among the actors in each series and each episode's advancing season storylines as part of the crossover. The Dominators storyline was regarded as its weakest link, with reviewers finding the alien race's motivations unclear. Ratings for the episodes were high, with each breaking records in viewership and the 18–49 rating and contributing to the most-watched week on The CW in six years. A fourth crossover, "Crisis on Earth-X", occurred the following year.
Plot
Barry Allen investigates a meteor crash outside Central City, which turns out to be a spaceship from which aliens emerge. Lyla Michaels tells the team that the aliens are known as the Dominators, and that they had landed on Earth during the 1950s, but mysteriously departed. Barry assembles the original members of Team Arrow, Thea Queen, the Legends, and Kara Danvers, Supergirl of Earth-38. The team begins training at a S.T.A.R. Labs facility, sparring with Supergirl to prepare for the aliens. Kara struggles to earn Oliver Queen's trust, and Cisco Ramon finds and reveals a message Barry's future self sent to Rip Hunter, which exposes Barry's manipulation of the timeline and its effect on other team members. As a result, only Oliver, Kara, Felicity Smoak, Martin Stein, Jefferson "Jax" Jackson, and Caitlin Snow still trust Barry.
After the Dominators abduct the President of the United States, the team goes to rescue him, with Barry sitting the mission out and Oliver staying behind in support of him. Kara leads the group, but the Dominators kill the President and activate a mind control device to take over the group. The controlled heroes return and attack S.T.A.R. Labs, where Barry and Oliver confront them. While Oliver holds them off, Barry lures Kara to the device and tricks her into destroying it, freeing everyone from the Dominators' control. When the team regroups, Ray Palmer tells Barry that everyone forgives and trusts him. Sara Lance, Ray, John Diggle, Thea, and Oliver are then abducted by the Dominators.
They are held in pods on a Dominator ship, each unconscious except for a shared hallucination of a simulated reality. In the hallucination, Oliver is living at Queen Manor, having never gotten on the Queen's Gambit, and is about to marry Laurel Lance. His parents are alive, and Diggle is operating as the Hood instead of Oliver. The captives begin seeing flashes of their real lives, and realize what has been done to them. Their escape attempt is blocked by personifications of their enemies: Malcolm Merlyn, Deathstroke and two of his Mirakuru soldiers, who killed Ray's fiancée Anna Loring, along with Damien Darhk and two of his H.I.V.E. soldiers. The adversaries are defeated, and the five awaken in the Dominators' ship and escape in a shuttle. Felicity, Curtis Holt, and Cisco try to hack into the Dominators' mainframe. Aided by Barry and Kara, they recover a device to locate the captives, who are rescued by Nate Heywood in the Legends' timeship, the Waverider. Aboard the Waverider, Ray deduces that the Dominators were gathering information about metahumans, using the hallucination as a distraction, to help them build a special "weapon". Meanwhile, the Dominator mothership heads towards Earth.