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 way Barry asked for Team Arrow's help is very similar to the way he asked them on the previous crossover, "Legends of Today". 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.
Barry and Cisco both mention the events of "Worlds Finest", 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 "Rogue Air", 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.
The message from future Barry, which first appeared on "Shogun", is finally revealed.
The message is later revealed to be a warning about Savitar, an evil future version of Barry.
Diggle finally finds out that his son J.J. was a result of Flashpoint, which Barry and Felicity found out in "Paradox". 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.
Caitlin tells Stein that she attacked her friends using her powers, referencing the events of "Killer Frost".
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 "Fast Enough".
In the Arrow season 2 episode "The Scientist" (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, "The Scientist" 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.
The Flash 3x08 Promo "Invasion!" (HD) Season 3 Episode 8 Promo - Crossover Event
The Flash Invasion! Scene The CW
The Flash 3x08 Sneak Peek "Invasion!" (HD) Season 3 Episode 8 Sneak Peek - Crossover Event