|"Things were looking a little grim, but part of me was, admittedly, a little excited for a "reunion" of old friends."|
- For other uses of "Invasion!", see Invasion! (disambiguation)
- "We're going up against a bunch of aliens, and you want to tell people that their lives might have been affected by time travel? One sci-fi problem at a time."
- —Oliver Queen
When aliens called the Dominators attack Central City, Barry speeds over to Star City to ask the Green Arrow for help in stopping them. Realizing the battle against the Dominators is bigger than they thought, they track down the Legends, and Barry and Cisco create a breach to bring back a secret weapon – Supergirl. Once all of the super friends are assembled, they come up with a plan to attack but a shocking secret related to Flashpoint is revealed and suddenly no one is sure who they can trust. Meanwhile, Wally wants to join the battle against the aliens but Iris tells him he's not ready.
The episode starts with Barry and Oliver in trouble. 10 hours earlier, in S.T.A.R. Labs, Barry, Iris, Joe, Caitlin and Cisco are running tests on Wally, who is exhilarated about his speed and wants to get on with the real action. However, Iris and Joe deny it, because they believe he's not ready. Joe also intends to do something concerning Alchemy and Savitar. H.R. calls to present his idea about opening S.T.A.R. Labs to the public. However, his demonstration is cut, when satellite picks up what seems to be a meteorite heading to Central City. Barry races to the impact site and discovers it's actually an alien spaceship. Number of aliens emerge, frightening Barry.
The next day, Lyla Michaels is on the site, but the site has been declared closed to the public. Barry meets Lyla and she tells him to meet her at S.T.A.R. Labs. There, Lyla tells Team Flash, that these aliens have been on Earth before, during the 1950s, and they called them the Dominators. Back then they came to collect intel on humans, and it ended in conflict. For some unknown reason they left, and now Lyla wants to find out if the Dominators are here again for intel, or for something far more nefarious. Lyla tells Barry to sit still, but he's not going to.
The scene changes to Star City, where Oliver and Diggle engage the Vigilante, when Barry suddenly speeds them out, before they are shot. They go to Felicity's apartment, and he informs them about the Dominators. Thea also joins. Barry also think about calling the Legends to help.
In a secluded aircraft hangar owned by S.T.A.R. Labs, Team Flash and Team Arrow wait for the Legends to arrive, which they do in the Waverider's jumpship. So far, the Legends contain Sara, Stein, Jax, Ray and Mick. Barry also intends to get an alien of their own for help, and he and Cisco open several breaches in order to reach Earth-38. They arrive in Kara Danvers' apartment in National City and Barry and Kara have a joyful reunion, and Kara immediately signs up.
Barry, Kara, and Cisco rejoin the others on Earth-1, and Oliver mistakes Kara as a human, until she demonstrates her ability to fly without additional aid, and using her heat vision to create the House of El-symbol on the ground. Kara calls each of Team Arrow, Team Flash and Legends with their own names, and they reply with their hero names. Barry brings everyone up to speed about the Dominators, and Kara explains what she knows about them. Barry is chosen to be the leader since he gathered everyone. Oliver suggests for everyone to train so that Supergirl is their practice target. Stein and Jax pull Barry to discuss about something, and Oliver comes along too.
Stein and Jax play a recording, which they found in a secret compartment of the Waverider. The message was meant for Rip Hunter. In the recording Barry explains that a war is coming and abot his tampering with the timeline. To clarify this, Barry explains how he created the Flashpoint and explains some of the things that are new in the current timeline. Barry intends to tell everyone, but Oliver says they need to focus on the Dominators for now.
Others train by attacking Supergirl, but their attacks are virtually useless against her (with Thea commenting Kara is a real badass and Sara saying it's hot). Oliver pushes everyone to keep on going. Later, Cisco and Ray are talking, when Barry interrupts them. Barry asks about Snart, and Ray reveals he sacrificed himself to save the others. Felicity notices that Cisco and Barry are tense, and Cisco admits he's mad at him, because his tampering with the timeline caused Dante to be dead. Stein asks Caitlin to go with him to see Clarissa, as Stein has memory changes, similar to what Barry experienced in Flashpoint. On the way Caitlin admits she is afraid of herself, due to Killer Frost inside her. They arrive at Stein's home, but instead of Clarissa, Stein meets Lily Stein, his daughter, whom he was unaware of.
Lyla is meeting with the president, and he agrees with her on the Dominators. As the president leaves in his motorcade, the Dominators suddenly attack and kidnap him. Meanwhile, Cisco has found the recording, and is even more mad at Barry, when he intends to tell everyone about the Flashpoint after the Dominator threat is over. Angered, Cisco demands that Barry explain everyone about, what he did. With no other choice, Barry tells everyone about the Flashpoint, pointing to Diggle that he used to have daughter instead of a son. Sara is angry at Barry too, because as much as she would like to return to the past to save Laurel, she can't, because of the fact what it would cause to the timeline, and Ray also points to Barry, how the Legends have been fixing time anomalies, only for Barry to create one on his own. They get a call from Lyla that the president has been kidnapped, and are called to action. However, nearly everyone are reluctant to trust Barry, save from Oliver and Kara. Barry decides to sit this one out and Oliver stays with him, leaving the rest to be led by Kara. As they leave Barry asks Kara, does she trust him. Kara says she will always trust him, but it's going to require a lot more to regain the trust of others.
Kara leads the team to an abandoned power plant. She scans the building with her x-ray vision, and sees the president, amazing everyone, especially Mick. Mick says he's not gonna call her Kara or Supergirl, but Skirt. The team enters the plant and encounter the president and one Dominator. Kara demands them to let the president go, but the Dominator claims he's not the one they want, and Diggle realizes they walked into a trap. The team is attacked and the Dominators activate a device, which affects all of them.
Inside the time vault in S.T.A.R. Labs, Barry watches the article about him disappearing in the future (the article is not written by Iris in this timeline), and Oliver shows up. Barry feels down for making the mistake of Flashpoint, but Oliver tells him that he can't keep beating himself for causing it. Oliver then tells him about what happened, after the Queen's Gambit sank. It was only him, Robert and a fellow crew member. Robert killed him and himself and told Oliver to survive. Then, he tells Barry how he and Thea watched his mother, Moira, to be killed by Slade Wilson. These events have made Oliver to want to undo all of that, pointing out, that it is what anyone would want to do, when they lose something dear to them. Meanwhile, Iris still tells Wally, he can't go into the field. Suddenly, the Labs fall under attack and Cisco calls Barry and Oliver to action. The two emerge, only to find their friends are attacking, as they have been mind controlled. Barry runs to the roof in order to engage Supergirl, Firestorm and Atom, while Oliver keeps Speedy, White Canary, Spartan and Heat Wave busy. Barry knocks out Firestorm and Atom, but Supergirl knocks him down. Wally decides to go to help as he thinks Barry and Oliver have no chance against the others, especially with Supergirl around there. He manages to knock out Thea, Sara and Mick, but Supergirl knocks him out cold. Barry rejoins Oliver, and Cisco and Felicity tracked a strange power source coming from the team's last known coordinates. Barry says whatever there is, is controlling them, and intends to lure Supergirl there, as her strength should be sufficient to destroy it. Barry leaves Oliver to hold off the rest while he goads Supergirl to see, which one of them is faster. Supergirl chases Barry across the city, and Oliver takes Wally to safety, but he runs out of arrows. He is forced to fight Sara (Ta-er al-Sahref vs. Al Sah-him) head to head. Barry manages to lure Supergirl to the site and positions himself in front of the device. Supergirl charges at him, but he uses phasing, which causes Supergirl to fly through him, destroying the device. The mind control is broken and everyone are back to their old selves. Kara apologizes to Barry, but he says she's not the first superhero to be mind controlled.
Iris and Joe scold Wally for what he did, and again deny him on going to the field. But H.R. offers to train him. The rest of the team are thankful for Barry and Oliver, while Kara scans the city for more Dominator devices. As Barry saved them, they are willing to stand with him. As they are leaving, Sara is suddenly caught in a beam, and she disappears. She is followed by Thea, Diggle, Firestorm, Ray and Mick. When beam hits Oliver, Barry attempts to save him, but he is too late.
- Martin Stein is revealed to have a daughter following the alterations to his personal timeline in "".
- The way Barry asked for Team Arrow's help is very similar to the way he asked them on the previous crossover, "". In both episodes, Barry saves Team Arrow members whilst they are in the middle of a mission, resulting in Diggle throwing up, before explaining why he needs their help.
- When asked what he saw, Barry describes that it was "enough to give Ridley Scott nightmares", referencing the director of the 1979 film Alien.
- This is the first appearance of Martin Stein and Jefferson Jackson on since Season 2's "" and Mick Rory since "". Ray Palmer returns after appearing in Season 1's "".
- It is revealed that Iris still has a crush on Oliver since "".
- The scene where Barry and Cisco recruited Kara was recycled from the episode "".
- Barry and Cisco both mention the events of "", particularly the fact that Barry and Kara's race was a tie.
- When Barry went to ask Team Arrow's help, Oliver asked him (rhetorically) if he needs help with "another evil speedster". This is a reference to "", where Oliver helped Barry battle the Reverse-Flash. Barry begins to answer that that is something he could use Oliver's help with (because of Savitar), only to change his mind and tell Oliver about the Dominators.
- Barry notes that Vigilante was about to "Scarface" Oliver and Diggle. This is a reference to the infamous final scene of the 1983 film Scarface, in which protagonist Tony Montana fires a hail of bullets at his enemies.
- Thea states that Barry claimed Kara to be "more powerful than a locomotive", referencing the traditional Superman catchphrase.
- Oliver mentioned his parents' deaths, which happened in "" (Robert Queen) and "" (Moira Queen).
- The message from future Barry, which first appeared on "", is finally revealed.
- Diggle finally finds out that his son J.J. was a result of Flashpoint, which Barry and Felicity found out in "". In the same episode, Barry tells Felicity about Cisco losing his brother as a result of Flashpoint. Felicity doesn't reply, giving a small hint that she already knew but didn't want to upset him.
- This episode marks the third time Oliver ran out of arrows during a fight. He previously did so in season 1's "" and season 4's "".
- The hangar at which the heroes gather is a reference to the Hall of Justice, a headquarters for the Justice League.
- The name of Supergirl's Earth is finally designated as "Earth-38", which is a reference to Superman's comics debut in 1938.
- H.R. claims that if he helped Wally to train, he would be "gone faster than humor in a Liam Neeson movie". This is a reference to how Liam Neeson tends to star in dark, gritty action films.
- Due to Flashpoint, the writer of the future newspaper is revealed to have changed; now being written by Julie Greer instead of Iris West-Allen. In the comics, Julie Greer is a reporter for KN News in Keystone City.
- When Barry asks Ray about Leonard Snart, Ray mentions Snart's heroic sacrifice at the end of "".
- Ray mentions to Cisco that his new A.T.O.M. suit is made of Old West Dwarf star, which the Legends collected following their adventure in "".
- Caitlin tells Stein that she attacked her friends using her powers, referencing the events of "".
- Cisco still can't forgive Barry after the revelations revealed during that episode and his actions trigger the later events in this episode.
- Wally runs at Mach 2, the same as Barry did in "".
- In the season 2 episode "" (which sees Team Arrow dealing with people who've gained superhuman abilities due to having been injected with Mirakuru), when Oliver tells Dig and Felicity that he's encountered Mirakuru-enhanced individuals before, he describes the Mirakuru as a serum designed to create human weapons. John then says "Human weapons? My God, what's next; aliens?" In this situation, that's exactly what Team Arrow (alongside Supergirl, Team Flash and the Legends) ends up dealing with.
- Furthermore, "" also introduced Barry Allen. Now, when Barry goes to ask Team Arrow to help him battle the aliens, John says "I swear to God, Barry; my life was somewhat normal before I met you"; a possible nod to the fact that John and the rest of Team Arrow first encountered strange things like Mirakuru soldiers around the same time that they first met Barry.
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