SPOILER ALERT: Do not read unless you have watched the Nov. 27 “Supergirl” and “Arrow” episodes, entitled “Crisis on Earth-X Part I” and “Crisis on Earth-X Part II.”

This year’s Arrowverse crossover — featuring the CW’s slate of DC series “Arrow,” “The Flash,” “Legends of Tomorrow” and “Supergirl” — was touted as feeling more like a four-hour movie than individual episodes and so far the episodes are on track to deliver.

The first part of the four-hour, two-night crossover was technically an episode of “Supergirl,” but unlike last season where the crossover action didn’t start in earnest until the tag at the end of the regular week’s episode, this one dives right in.

“Crisis on Earth-X Part I” starts on the Nazi-controlled Earth-X, where a masked, bow-wielding Nazi takes down a group of armed individuals with relative easy. Not even the Earth-X version of Guardian (Mehcad Brooks) is a match for the masked man, who enters a warehouse to take possession of a machine that was being held their by some kind of resistance.

Back on the much brighter Earth-One, Iris (Candice Patton) calls Barry (Grant Gustin) to inform him that there are still some people who haven’t RSVPed to their upcoming wedding. Those people include Oliver (Stephen Amell) who gets a call from Felicity (Emily Bett Rickards) to make sure they’re going, the Legends — particularly Sara(Caity Lotz), Dr. Stein (Victor Garber), Jax (Franz Drameh) and Mick (Dominic Purcell) — who realize they flat-out forgot to respond, and Kara (Melissa Benoist) who had decided not to go thanks to her current relationship woes with Mon-El (Chris Wood). Thankfully, she decides to get up off the couch and head to Earth-One with her sister Alex (Chyler Leigh) — who is struggling with her own relationship problems — as her plus-one.

Once everyone arrives in Central City, Iris, Caitlin (Danielle Panabaker), Kara, and Felicity head for a day of pampering and champagne before the rehearsal dinner. Iris asks Felicity if she thinks marriage is in the cards for her and Oliver. She explains that once all the craziness in their lives settles down that it could be a possibility.

Meanwhile, Barry and Oliver are getting fitted for their suits and having a very similar conversation. It’s always a treat when Gustin and Amell are onscreen together, and this scene especially so because Barry gets the chance to give Oliver advice rather than the other way around.

“People like us are always going to be getting into trouble,” Barry explains. “Having someone we love by our side just makes getting out of that trouble that much easier.”

Over on “Legends of Tomorrow,” an ongoing story has been Stein and Jax’s ongoing struggle to find a way to separate Firestorm — partially to make way for Garber’s exit from the series — so that Stein can spend time with his family. Thanks to Cisco (Carlos Valdes) and Wells (Tom Cavanagh), they finally have a way to do it. Stein seems enthused but notices his longtime partner seems less than excited to lose his superpowers.

The episode’s rehearsal dinner serves as a great way to get characters that don’t normally get to interact with each other a chance to do so. A couple of the highlights are Sara and Alex getting wasted and ducking out of the event early with each other, as well as Mick asking Caitlin if he kidnapped her before and her warning to him not to try it again.

Stein arrives late and explains to Jax that he created a serum to give him the powers of a spider — but only the ability to stick to walls. Jax walks away mad, saying Stein doesn’t get why he’s mad.

However, the biggest moment from the rehearsal dinner comes when Felicity turns down Oliver’s marriage proposal — loudly and stealing the spotlight from her betrothed friends for a moment. The festivities had clearly gotten to Oliver, who decided to try for his second proposal to Ms. Smoak. Felicity explains she’s down for everything that marriage entails — except for the legal part. Oliver protests, but Felicity will not budge in her stance.

With two relationships (Stein/Jax and Oliver/Felicity) heading through turbulence and Alex freaking out about her one night stand with Sara, the West-Allen wedding has a bit of tension in the air. Shortly before the wedding begins, Barry is confronted by a waitress (Jessica Parker Kennedy) who tells him how excited she is for him and that she thinks the marriage will be “one for the ages.” She definitely seems to know him despite claiming she doesn’t.

It wouldn’t be a superhero wedding without it being crashed by some sort of villain — in this case it’s Nazis. Before they can say their vows, Barry and Iris are interrupted by Nazi troops, the masked man with the bow, a flying woman and a man with a sword. What follows is a great fight scene that really displays why these crossovers are so fun for fans as different groups team-up to take down the enemies. While Supergirl fights the other equally-strong flyer, Oliver and Cisco work together against the masked bowman, Sara and Alex manage to stop the man with the sword, and Heatwave and Killer Frost fight to take down soldiers.

After Supergirl takes down the Earth-X flyer, the Nazis retreat — aside from the masked swordsman, who the heroes lock up beneath S.T.A.R. Labs. The episode ends with the Earth-X Reverse Flash (Cavanagh) showing up to scold the other two, which it turns out, to the surprise of no one thanks to all the photos and trailers already released, is the Earth-X version of Supergirl and Green Arrow. (Benoist and Amell pull double duty to play their evil dopplegangers.) The episode ends with evil Supergirl claiming they’ll kill every last one of the heroes.

Part 2 of “Crisis on Earth-X” begins with quite the shocker, though. Prometheus — the Earth-X attacker that Sara and Alex took down at the wedding — takes off his mask to reveal not Adrian Chase, but Tommy Merlyn (Colin Donnell). Last year “Arrow” couldn’t lock down Donnell for the show’s 100th episode, but they managed to make good by including him here.

Oliver and Tommy-X have a short conversation where the former tries to convince the latter that there is still good in him. Tommy-X assures Oliver that the world he comes from is stronger and will take Earth-One, but that he regrets not being able to see it happen. Then he kills himself with poison.

Out in the cortex, the rest of the heroes learn from Wells that Earth-X is the 53rd Earth in the multiverse that never received a designated number because it was so horrible nobody would ever travel there. On Earth-X the Nazis won WWII and Hitler went on to take control of the world until his death in 1994.

The rest of the Earth-Xers have a quick squabble following the death of Tommy-X. After their argument is resolved, Supergirl-X comforts Oliver-X and says she trusts him with all of her heart, then they kiss.

Despite all these Nazi doppelgangers, Oliver still finds time to confront Felicity about her not wanting to marry him. He asks her if it was the spontaneous way he asked her that made her react the way she did. She explains that she doesn’t want to be married because the last time she was ready to get married she was shot and paralyzed — and when Barry and Iris tried their wedding was crashed by Nazis.

“We have something so great going,” she explains. “Let’s not change that.”

Oliver says that he loves her but he wants to move forward and marry her. Felicity says she loves him too, but asks him to respect her on not wanting marriage. Later, and with some privacy, Felicity confides in Iris that the last time she and Ollie got engaged she was shot and that was the beginning of the end for their relationship. She doesn’t want to tempt fate with that happening again.

Oliver and Felicity aren’t the only two to pick a questionable time to work through their issues. Stein speaks to Jax about what happened at the rehearsal and Jax explains that it isn’t losing his powers, but losing Stein that has him upset.

“I never knew my father,” Jax explains. “But I got to know you. You’re the closest thing I’ve got to a dad, and now you’re leaving.”

When the Earth-Xers attack a facility in the city, Green Arrow, Supergirl and Flash head off to stop them. There they finally learn about their doppelgangers and that Earth-One’s Eobard Thawne has allied himself with Earth-X. We also learn that Green Arrow-X and Supergirl-X aren’t just dating but married. After another quick fight the Xers get away with what they stole — a device we later learn can be converted into a neutron bomb.

Thanks to the kryptonite arrow with which Oliver hit Supergirl-X during their fight, the team — minus Iris, Felicity, Mick and Caitlin — head off to stop the doppelgangers. While the majority of the team is distracted with the Nazis and their ambush, Oliver-X breaks into S.T.A.R. Labs. While Felicity and Iris hide, Mick and Caitlin prepare to take on the doppelganger. They get a bit of help when the rest of Team Arrow — Mister Terrific (Echo Kellum), Black Canary (Juliana Harkavy) and Wild Dog (Rick Gonzalez) — show up. But even all of them together can’t take him down.

The team at the docks gets taken down by the Nazi ambush — they get tied up and affixed with power-dampening collars. Oliver-X arrives just as Supergirl-X collapses. Like Icarus, she flew too close to the sun and now has radiation poisoning. But Oliver-X isn’t worried because they can heal her dying body with Supergirl’s heart. So they stand around and chat about the predicament for a few minutes before Earth-X’s Metallo shows up.

Metallo knocks out the heroes, and when they wake up they realize they are imprisoned — on Earth-X!

“Crisis on Earth-X” concludes with Parts III and IV Tuesday, Nov. 28, starting at 8 p.m. on the CW.