Picking up where Part 2 left off, John Deegan's used the Book Of Destiny to alter reality again. Barry & Oliver are now the Trigger Twins, a pair of wanted criminals. They flee to a rooftop, where they're confronted by what appears to be a black-clad Superman. Oliver realizes Superman's really Deegan, who's used the Book to transform himself into a superhero (who I'm going to call "Duperman," to differentiate him from the real thing).
Duperman tells the pair that The Monitor gave him the Book once again, and he's rewriting reality in a big way this time. He gloats as he tells Barry & Oliver he's altered all their family and friends, and boasts that he's now the beloved hero of this world. Barry tries to tell him there's more to being a hero than wearing a costume.
Oliver shoots the cables on a construction crane, causing it to fall off the roof and endanger a group of kids far below. He tells Duperman to make a choice— save the kids or capture him and Barry. Duperman grudgingly saves the kids, allowing Barry & Oliver to escape.
Cut to STAR Labs, which is now Duperman's HQ. It's staffed by an army of secret police, including John Diggle and Killer Frost. Duperman returns, furious that Barry & Oliver got away. The others nervously praise him, desperately trying to pump up the unstable "hero's" fragile ego.
Duperman enters the STAR Labs Secret Super Jail, where we see he's holding Kara captive. He says there's nothing in the Book about her, as if she's "from another world." Hmm... I thought the Book contained knowledge of everything? Maybe he skipped the chapter on the multiverse.
Anyway, he tells her he's keeping her locked up until he can figure out what she is. In the meantime, he explains that he's transformed himself into a male version of her, who just happens to look like the real Superman of Earth-38. None of this makes the least bit of sense, so let's just move on.
Kara tries to escape the cell, but she's stopped by Alex Danvers, who uses a sonic device to punish her. Apparently there's an Earth-1 version of Alex, and she's it? Or maybe the Book just up and created a new version, who knows? Anyway, Kara's shocked to see her sister exists in this reality.
Meanwhile, Barry says they need to track down Cisco so he can vibe them to Earth-38. Once there, they can find the real Superman and bring him back to fight Duperman. They arrive at Cisco's apartment, but when they enter they find themselves standing on a platform in outer space. The Monitor's there, and spouts a bunch of hooey about self-sacrifice and destiny, telling them it won't be enough to defeat what's coming. He transports them back to Earth-1.
Barry reasons that Cisco's probably a bad guy in this reality, so they look for him in a criminal hangout. There they encounter Time Bureau Agent Gary Green, who's a bartender in this world. He shows them where to find Cisco. Unfortunately, "Mr. Ramon" isn't pleased to see them, and has his enforcers— including James Olsen— rough them up.
Barry mentions Cisco's brother Dante, which piques his interest. He meets privately with them, and asks why he shouldn't just have them killed. Barry tells Cisco he knows about his vibing power, and can teach him how to breach to other Earths.
Back at STAR Labs, Kara overhears Alex and Killer Frost talking outside her cell. She sees that Alex is pretending to be straight in this reality, and realizes she can use that to her advantage.
Cut to the Fortress Of Solitude on Earth-38, where Superman and Lois Lane are hanging out. A breach opens, and Barry, Oliver and Cisco step out. How'd they know where to find Supes on this entire planet? Your guess is as good as mine.
Anyway, Barry explains that Deegan has altered reality even more extensively this time. Kara's nowhere to be found, and Deegan's turned himself into an evil version of Superman. He says Duperman's likely storing the Book somewhere inside STAR Labs, and Superman says they need to get it back.
Kara sees Alex walk past her cell and calls to her. She goes on and on about how totally cool and awesome the Earth-38 Alex is, and how they're sisters on her world. Amazingly, it takes literally thirty seconds to win the trust of this version of Alex. She opens Kara's cell and they attempt to escape.
Unfortunately they're stopped by Duperman, Diggle and Killer Frost. Duperman's about to incinerate them with his heat vision, when suddenly a breach opens and Superman, Barry, Oliver and Cisco emerge. Duperman attacks Superman, and the two fly through the ceiling and into the sky. Oliver says he'll take care of Diggle and Frost, and for some reason the three of them board an elevator for their fight.
Kara asks Alex where Duperman would keep something valuable like the Book. She says most likely in his Fortress Of Solitude. Barry complains that he just came from there, but they discover that in this reality, Duperman's Fortress is actually the Time Vault inside STAR Labs.
Kara kicks open the vault's door and grabs the Book. She opens it and tries to use it to restore reality, but she's too weak from being in the Super Jail (?). She says they need to get the book to Superman.
Meanwhile, Superman and Duperman battle it out in the skies over Central City. They crash though an office building, and Duperman throws Superman through a window. He lands on the street far below, and Duperman prepares to murder him.
Suddenly Kara appears and uses her heat vision to blast Duperman into the next county. She hands the Book Of Destiny to Superman, who opens it and begins reading it (I guess?). Swirls of blue energy emanate from the Book, as Superman begins restoring reality.
Kara's caught up in the reality wave, and transformed back into Supergirl. Inside STAR Labs, the wave finally restores Barry's costume and speedster powers. The elevator opens up, and Oliver appears, dressed as Green Arrow once again. He tosses out an unconscious Diggle and Killer Frost.
Barry zooms outside and watches in awe as Superman resets the world. Suddenly Duperman appears behind Barry and grabs him, threatening to break his neck unless Superman hands over the Book. What are we watching here, Man Of Steel?