Every year the superhero crossover on The CW grows larger, and this season it’s practically bursting at the seams. Not only are “Arrow,” “The Flash” and “Legends of Tomorrow” involved, but “Supergirl” is joining the fun, thanks to its big move to The CW.

With four shows telling a story over three episodes, there’s a lot to cover. From aliens to alternate timelines to inter-dimensional travel, practically nothing is off-limits with the three-part “Invasion!” event. That’s why this handy guide might be exactly what you need.

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Who are they fighting against?

Who is a formidable enough opponent to bring together the superheroes of four different shows? The baddies in this crossover are an alien race known as the Dominators. These villains are pulled from the “Invasion!” comic storyline, armed with both advanced technology and the ability to alter their own genetics.

While the quick glimpses at the Dominators show us their look is definitely inspired by their comic book counterparts, it’s unclear if they’ll have the same powers. Regardless, the Dominators are clearly enough of a threat to need a rival alien from another dimension.

How does Supergirl get to Barry’s world anyway?

Cisco’s Vibe powers are a lot stronger. So strong, in fact, it seems hey can find Kara’s Earth and go there. That leads Barry and Cisco to head to Supergirl’s planet to bring her back and fight in the clip below:

Cross your fingers we’ll get at least a taste of what happens when Cisco meets Winn, though.

Does Flashpoint have something to do with this?

The official episode description for “The Flash” says a “shocking” Flashpoint secret will be revealed, leaving the heroes to wonder who they can trust. What’s more, “Arrow” seems to be occurring on a much different timeline — one which Oliver’s parents are alive and he’s engaged to Laurel.

Something definitely isn’t right here… And if we had to guess, Flashpoint may be at the root of it.

Wait, Laurel is alive?

Yes and no. In this alternate timeline Oliver and his dad were never lost at sea, so his family’s intact and he’s engaged to Laurel, Felicity is with Ray and the Lance sisters seem closer than ever.

This is “Arrow” though, so the happy new world just isn’t going to last. Don’t get your hopes up that Laurel is going to be returning to Star City permanently.

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What about Killer Frost?

We’re in for a one-week Killer Frost reprieve. There’s still plenty of Caitlin Snow’s story to tell, but for now there’s bigger things to focus on — like the world being conquered by aliens.

Superman, maybe?

Calm down. While it’s entirely possibly that Superman will figure into a future crossover, he’s not even remotely involved in this one. It’s Supergirl’s year and her time to shine.

With so any moving parts, just how hard is this to pull off?

Very. While it helps that all four shows film in Vancouver, home to the multiverse, creating these crossovers is still a huge undertaking for all involved — including the actors who suddenly have a lot more work to get done in a very short amount of time.

“It is a four-way crossover insofar as it begins at the end of ‘Supergirl’s’ hour, but it really was her as a visitor, and then a three-way crossover between ‘Arrow,’ ‘Flash’ and ‘Legends’. So everyone got together this year and they made a schedule,” Stephen Amell reveals. “Our first A.D., who sort of put together that schedule, Ken Shane, handed me this big piece of paper that was just five straight weeks of where I was going each day. It was hard work, I mean I didn’t have a day off for five weeks, but there was a spirit of teamwork this year that made it palatable.”

And that doesn’t just go for Amell, who’s celebrating the 100th episode of “Arrow” during the crossover but the CW’s latest edition as well:

“Within the chaos, that’s what we all love about it,” “Supergirl’s” Melissa Benoist says. “When do you get to do something like a crossover with not just one show, but there’s three shows. I think, logistically, it’s kind of an impossible feat that we somehow did in terms of scheduling.”

So while the week of crossovers may pass quickly, never forget just how big an undertaking something of this caliber is.

The “Invasion!” crossover kicks off with “Supergirl” Nov. 28, at 8 p.m. ET/PT on the CW.

Posted by:Chris E. Hayner

Chris E. Hayner is equal parts nerd, crazy person and coffee. He watches too much TV, knows more about pro wrestling than you do and remembers every single show from the TGIF lineup. You may have seen him as a pro-shark protester in "Sharknado 3." His eventual memoir will be called "You're Wrong, Here's Why..." TV words to live by: "I'm a firm believer that sometimes it's right to do the wrong thing."