“Crisis on Earth-X, Part 1” starts off with a very dark and dreary shot of Earth. Nazi propaganda pieces with swastikas are draped over the city’s skyscrapers.
At a warehouse, the Earth-X version of Green Arrow takes the warehouses guards before he’s interrupted by the Earth-X Guardian/Jimmy Olsen. After a brief battle — one in which Jimmy displays a U.S.-themed shield — Arrow-X kills Jimmy.
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Back on Earth-Prime, Barry is in the midst of a fight with King Shark when Iris calls in on the communication system, frustrated that not everyone has RSVP’ed to their wedding.
In Star City, Oliver is in the midst of his own fight with a bunch of thugs when Felicity asks him whether they’re going to the Allen wedding, Oliver says that they are.
In 1183 England, the Legends are busy taking on some knights, and Sara asks the rest of the Legends if they’ve remembered to RSVP’ed to the wedding.
In National City, Supergirl is fighting a Dominator. Mon-El shows up to the DEO to make sure Kara is all right. Later that night, Kara and Alex are talking about Mon-El at Kara’s apartment, and Alex finds Barry and Iris’ wedding invite. Kara explains that it’s probably best if she doesn’t go to the wedding, but mulling it over further — Kara decides that they’re going to get their mojo back and attend.
Act II
Iris and her bridesmaids — Caitlin, Felicity, and Kara — are getting their nails done when Iris asks Felicity if she thinks her and Oliver will be next.
Barry’s getting ready in his tux as Oliver helps him. Barry asks Oliver the same question, and tells Oliver to “put a ring” on Felicity as costumed heroes will always get in trouble.
At STAR Labs, Jax and Stein have approached Cisco and Wells about separating Firestorm. Cisco and Wells reveal that they’ve used quarkspheres to come up with a serum that, once taken, will render the taker unable to use their Firestorm powers. Stein is excited at the opportunity to live a normal life while Jax — who now knows that separating Firestorm is a reality — isn’t as excited as his counterpart.
Act III
Caitlin approaches Stein, and the two reminisce about Ronnie. Caitlin runs along to the wedding while Stein stays behind to examine a vial of the serum Cisco and Wells worked on.
Everyone has gathered at Jitters, where Barry and Iris are holding their rehearsal dinner. While there, Sara and Alex begin chatting and start sharing drinks.
Upstairs, Barry starts talking to Kara and Mon-El comes up. Kara informs Barry that Mon-El is now married to another person, and the two switch the conversation. Barry asks Kara for a favor.
Stein walks into Jitters late and pulls Jax aside. He explains that he reworked the serum that Cisco and Wells worked up into something that would allow Jax to still have powers. He explains that they’d be Spider-Man-like powers instead of Firestorm powers, and Jax gets somewhat frustrated that he won’t remain Firestorm.
Joe starts his toast and gives a heartfelt thanks to Barry and Iris, thanking them for showing him how to love again.
The camera pans outside to show Sara and Alex making out.
Oliver and Felicity are talking about having a rehearsal dinner, and when asked why they’d have one, Oliver informally proposes to Felicity. She quickly turns it down, explaining that they don’t need a piece of paper to show their love to each other, and when the two start quietly arguing, Felicity ends up yelling that she doesn’t want to marry Oliver, which makes everyone else at the rehearsal dinner turn around.
Act IV
All of the characters begin waking up the next day. Barry wakes up with a smile on his face. Oliver and Felicity each wake up frustrated, and Alex wakes up in bed with Sara.
Barry speeds back, and he’s prepared a breakfast for Iris, leaving her a note that says it’s bad luck for the bride to see the groom on their wedding day.
Everyone’s congregating at the church, and Kara is waiting outside for Alex, who shows up incredibly hungover. Sara shows up shortly thereafter, and an awkward moment ensues between her and the Danvers sister where Kara finds out the other two got intimate.
Barry’s approached by a waitress, who offers him some sparkling water before explaining her excitement about being able to be at the wedding. Barry is taken aback by the conversation, which is rather awkward.
Kara begins singing as Joe walks Iris down the aisle and the wedding ceremony begins. When the pastor asks if someone opposes the wedding, he’s instantly vaporized by Overgirl, who’s shown up on Earth-Prime with her Nazi army.
Act V
The Nazis begin opening fire as the heroes evacuate the church. Kara and Overgirl start battling in the sky over the church while Sara and Alex start taking on the Earth-X version of Prometheus.
Caitlin turns into Killer Frost and teams up with Mick to take down some of the Nazis inside while Oliver starts battling Dark Arrow.
Kara is knocked back inside by Overgirl, but the former quickly takes out her evil counterpart. Seeing Overgirl knocked unconscious causes Dark Arrow to call the soldiers to fall back to Earth-X.
Act VI
The group rendezvous at STAR Labs, and we find out the Cisco has been knocked out and is suffering from a minor concussion. The group was able to capture Prometheus-X, and Jax and Sara escort him to a holding cell in the bottom of STAR Labs.
The group gets together to discuss their next steps, and Oliver steps forward to say they should make Prometheus-X talk.
On Earth-X, Dark Arrow and Overgirl gather on a rooftop, and Dark Flash speeds in yell at the pair for their failed mission at the wedding.