The four-series crossover between Supergirl, The Flash, Arrow and Legends of Tomorrow begins tonight. We cover five things to know to get ready for the four-night event.

The team will face off against the Dominators

“This year, for our mega ‘Arrowverse’ crossover, we’re taking inspiration from a DC crossover from the late 1980s known as Invasion!, which featured an alien race called the Dominators, who’d previously vexed the Legion of Superheroes,” EP Marc Guggenheim previewed in September.

“We’re using cutting edge prosthetics and computer effects to achieve a feature film-quality look which is faithful to Invasion! artist Todd McFarlane’s interpretation of the characters.”

The Dominators, collectively known as the Dominion, are an alien race from the farthest reaches of the universe. A technologically-advanced species who specialize in genetics, their society is extremely hierarchical, with status depending on the size of a red circle that appears on their foreheads.

Invasion! was a major, three-issue crossover event in 1989 that featured the Dominators joining an Alien Alliance to wipe out humanity due to the threat the meta-gene that allows for humans to develop superpowers presents.

The CW released a video of found footage showing that The Dominators have been to Earth before this crossover:

‘Supergirl’ will be mostly standalone

Though the big Heroes vs. Aliens crossover is being teased as a four-series crossover, the main plot of the Supergirl episode, titled “Medusa,” will deal with the Supergirl world. Barry and Cisco from The Flash will appear to ask Supergirl for her help on their Earth, but they will not be the main focus of the episode.

The synopsis for the episode reads:

Eliza (guest star Helen Slater) comes to town to celebrate Thanksgiving with her daughters. Alex (Chyler Leigh) decides it’s time to come out to her mother; Kara (Melisa Benoist) is shocked when Eliza suggests Mon-El (Chris Wood) has feelings for her, and Winn (Jeremy Jordan) and James (Mehcad Brooks) consider telling Kara the truth about the Guardian. Meanwhile, CADMUS unleashes a virus that instantly kills any alien in the vicinity so Kara enlists an unexpected ally – Lena Luthor (Katie McGrath). The epic superhero crossover kicks off tonight when Barry Allen (guest star Grant Gustin) and Cisco Ramon (guest star Carlos Valdes) seek Kara’s help with an alien invasion on their Earth.

Some characters will react better to Supergirl than others

Sara Lance will, according to EP Andrew Kreisberg, be “instantly charmed” by Kara. “I think Sara’s got a little crush on her,” he tells TVLine.

Meanwhile, Diggle will have a typically-amusing response. “There’s a moment where somebody says, ‘This is Supergirl,’” Kreisberg describes, “and they say, ‘What’s so super about her?’ and she just lifts off the ground. Dig’s like, ‘I’m convinced.’”

Watch the clip of the aforementioned scene below:

Oliver, however, will be less enthusiastic. According to Kreisberg, “Part of Oliver’s journey in those [crossover] episodes is he’s kind of hit his breaking point [in terms of] things he’s willing to accept.

“He’s had Mirakuru soldiers and the Lazarus Pit and magic and Damien Darhk. Flying aliens who can incinerate you with their eyes, it’s one beyond the pale for him. He’s keeping his distance, which is a very Oliver thing to do. Kara sort of comments, ‘Does he not like me?’ and Barry’s like, ‘No, that’s kind of how he treats everybody.'”

‘Arrow’ will celebrate its 100th episode

We’ve been hearing that quite a cast of former Arrow characters will be returning for the 100th episode, but it’s been unclear how the likes of Moira and Laurel would fit in.

Now we’ve learned that they will be appearing in a dream sequence as, according to the synopsis, “Oliver 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 imminent. Everything seems perfect, but Oliver starts to notice small imperfections that make him question this new reality.”

It sounds very similar to the Supergirl episode “For the Girl Who Has Everything.”

The 100th episode “is such a radical departure from anything that we have ever done on the show,” Stephen Amell tells TVLine. “We utilize the fantastical elements that we bring in on The Flash and carry through to Legends of Tomorrow with the present-day storyline to create something… that is not a flashback [or] a flash-forward. It’s something else entirely.”

Amell adds that the dream sequence “basically gives us license to do whatever we want and to show some things that we’ll never be able to show, to have some characters interact that will never be able to interact on the show again — probably.”

Additionally, “There [is] a very therapeutic moment for my character towards the end [of the Arrow episode] where I get to say goodbye to people that I never got the chance to say goodbye to,” Amell teases.

We’ll learn more about Barry’s message from 2056

In discussing future Barry’s message, Victor Garber tells Comic Book Resources, “It sort of comes out in dribs and drabs. It’s a big mystery and a big deal.”

Moreover, he teases that Barry will find out about his future self’s message to Rip Hunter: “[I]t’s just part of the whole mystery of time travel and the fact that this is coming from Barry years later and then when he finds out — and that all happens in the crossover, so it just sort of unfolds as these things do on these shows, and lots of things occur in that time period.”

Bonus: Watch the extended trailer

Will you watch the crossover between ‘Supergirl,’ ‘The Flash,’ ‘Arrow’ and ‘Legends of Tomorrow’?