“Crisis on Earth-X, Part 4” begins with the team trying to take down the Nazis immediately surrounding Stein in an attempt to save him. Unable to get any closer, Stein decides to begin crawling towards the power switch. He’s able to climb up and flip the switch, but is shot again in the process.
Outside, Barry is able to throw a lightning bolt and combine it with The Ray’s powers to fry Red Tornado before he flattens the bunker.
Back inside, Jax and Stein merge back into Firestorm, and when Barry and The Ray arrive, the team goes through the breach back to Earth-1.
Thawne is starting the procedure back up when his tool is stopped by a miniature man — it’s The Atom. He grows to normal size and knocks out Thawne, stopping the procedure.
Steel frees those trapped in the pipeline while Amaya and Zari clear out the remaining Nazi guards. Thawne walks in to tell Dark Arrow that it’s not safe on Earth-1 anymore, and the two leave as the group previously stranded on Earth-X breaches back to Earth-1.
Team Arrow and the Legends are stopped by Metallo while Kara, Felicity, and Ray are approached by Dark Arrow, who is in turn distracted by Oliver holding Overgirl hostage with a knife.
The Legends and Team Arrow combine their powers to best Metallo while Thawne speeds in and rescues Overgirl from Arrow.
On the Waverider, Jax and Stein separate, and it’s revealed that although Jax hasn’t been hit, he’s able to feel all of Stein’s injuries.
Mick grabs a beer and is soon approached by Snart, who Mick thinks is still a ghost. Citizen Cold tells his Earth-1 partner that Heatwave’s doppelganger on Earth-X has died.
Back on the Waverider, Caitlin walks out of the medical bay and tells Ray that Stein shouldn’t even be alive right now — Jax is serving as a human life support machine. If they don’t do something, they’re both going to die.
Amaya, Zari, and Curtis are introduced to The Ray, and that quartet tries to locate the Nazis’ version of the Waverider.
Back in the medical bay, Stein wakes up and tells Jax he’s nothing more than an anchor around Jax’s neck. Stein tries to convince Jax to let him take Cisco’s serum, which would save Jax and kill Stein in the process. Jax hands Stein the serum, and the latter takes it, and the two stay together until Stein passes away due to his injuries.
Sara walks in to find that Stein has passed away, and word quickly spreads throughout the ship.
The various heroes embrace and start to grieve while Jax goes to Stein’s household to inform his wife and daughter about Martin’s passing.
On the Nazi Waverider, Dark Arrow continues to support Overgirl, who’s suffering drastically by now.
Everyone gathers on the bridge of the normal Waverider to begin discussing strategies. Dark Arrow calls the ship to draw a truce: they’ll leave Earth-1 alone if the team hands over Supergirl.
Oliver rejects the offer, and the team gets ready for battle.
Furious over the rejection of his offer, Dark Arrow and his Nazi cronies begin opening fire on the citizens of Central City. Before too long, the Arrowverse heroes show up and an all-out brawl breaks out.
In the air, Felicity, Iris, Cisco, and Wells have stayed behind on the Earth-1 Waverider and soon begin pursuing the Nazi Waverider, trying to shoot it out of the sky.
Killer Frost, Amaya, and Zari are able to board the Nazi Waverider to shut it off, and Cisco vibes the heroes out of the ship so they don’t get hurt in the impending crash.
Supergirl and Overgirl are battling in the sky over Central City while Barry and Thawne race around. Barry’s eventually able to stop Thawne, but refuses to kill him. He allows Thawne to leave, but not without a warning from Thawne that he’ll return.
Gideon informs the group on the Earth-1 Waverider that Overgirl is about the turn into a supernova, and Kara flies her into space so she doesn’t harm anyone. Distracted by her death, Oliver is able to overpower and kill Dark Arrow.
Unconscious from the blast, Kara is about to crash land in Central City but is caught by a steeled-up Nate, saving her.
After the battle, Cisco opens a breach so that The Ray and Citizen Cold can travel back to Earth-X, but Citizen Cold decides to stay behind on Earth-1 for reasons unknown.
The team gathers at Stein’s funeral, and Jax delivers the eulogy.
After the funeral, Alex and Sara part ways, but not before Alex is able to thank Sara for giving her perspective on life. Sara leaves and the Waverider heads through time.
Barry and Iris reveal that they’re not going to have a large ceremony for their wedding,
Felicity mentions that they know someone who’s ordained and Barry speeds off. When he returns to the Central City waterfront, it’s revealed that Diggle is ordained and can legally perform weddings.
Oliver acts as best man while Felicity serves as maid of honor and Barry and Iris begin to exchange their vows. Towards the end of the vows, Felicity interjects and asks Diggle to marry her and Oliver as well.