Full spoilers up ahead for the third part of The CW's mega-crossover event "Crisis on Infinite Earths!"
"Crisis on Infinite Earth: Part 3" begins with a good chunk of the Arrowverse reuniting on the Waverider. Team Flash has arrived and is filled in on what's going on, leading Cisco to help Ray Palmer fix the Paragon-finding device. After that's done, it's revealed the only unknown paragon is a character by the name of Ryan Choi. Ray, Iris, and Ralph go after Choi, who's too afraid to help them out and leaves to go home to be with his wife and kid. Iris tracks him down and talks to him about the Crisis and convinces him to join the team.
Shortly thereafter, Diggle finds out of Oliver's death and subsequent reanimation — he agrees to go to Purgatory with Sara in an attempt to coax Oliver's soul back into his body. This leads Diggle, Mia, and John Constantine to Earth-666 where they run into none other Tom Ellis' Lucifer from the Netflix show of the same name. Using some pointers from Lucifer, the trio is transported to Purgatory — which looks just like Lian Yu — and tracks down Oliver's spirit. As they're about to return Oliver's soul to his body, a mysterious man named Jim Corrigan stops them. After they talk, Corrigan seemingly convinces Oliver to take up the mantle of The Spectre. Diggle, Mia, and Constantine are teleported back to the Waverider.
Elsewhere, The Monitor decides Cisco plays a role in the grand scheme of things and gives Cisco his vibing powers back, despite the latter's concerns. Cisco joins with Killer Frost and Barry at the location where the Anti-Monitor surfaced in hopes of imprisoning him again. They eventually get into the character's hidden lair and find the Earth-90 Flash (John Wesley Shipp) trapped in a constant run on a treadmill. It's revealed the Anti-Monitor has locked the elder Flash on the treadmill to power his anti-matter cannon that's in the process of destroying the multiverse.
They manage to get Shipp's Flash off the treadmill, but he reveals the Anti-Monitor put in a fail-safe — if he stops running, the multiverse will be destroyed at once. This causes Pariah to disappear and return moments later with Jefferson Pierce/Black Lightning. The Freeland native tries absorbing the energy of the cannon but is unable, which causes Barry to make a drastic decision — he must disappear in the Crisis.
As Barry's about to sacrifice himself, Earth-90 Flash stills Barry's speed and chooses to sacrifice himself. Earth-90 Flash manages to get back on the treadmill and overload it with speed, destroying the cannon. He's vaporized in the process and the rest of the group is teleported out just in time.
Throughout the episode, Kara continually flirts with the idea of using the Book of Destiny in an attempt to restore Earth-38. At every turn, Kate Kane is there to stop her.
The entire group reunites on the Waverider and tries to figure out how to find Lyla — at that instant, she teleports back to the ship. We find out the Anti-Monitor is controlling her and she begins attacking Diggle. One by one, the heroes try taking her down but they're all thwarted. This leads to the Monitor putting his life on the line and sacrificing himself. Anti-Monitor then uses a new wave of anti-matter to consume Earth-1. In the nick of time, Pariah teleports the seven paragons to an alternate reality where they'll be safe from the destruction of all life.
The Crisis on Infinite Earths event returns in January.