WARNING: The following contains spoilers for What If...? Season 1, Episode 9, "What If... The Watcher Broke His Oath?," streaming now on Disney+.
With the fate of the multiverse in jeopardy, the Watcher makes an impossible decision in What If...?'s Season 1 finale. "What If... The Watcher Broke His Oath?" finds the omnipotent being seeking out the multiverse's greatest heroes in an effort to stop Ultron from ending each and every reality.
The episode kicks off with Captain Carter, who's experiencing the events of Captain America: The Winter Solider in her reality. After boarding the Lemurian Star, she finds and begins to battle Batroc, but their fight is cut short by the Watcher coming to recruit her. The Watcher then heads to the reality where T'Challa became Star-Lord to pick up the Wakandan prince, who has just saved Peter Quill from the clutches of his father, Ego.
The next destination on the Watcher's trip is an unfamiliar one. Journeying to Nidavellir, he finds Tony Stark and Gamora working to build an Infinity Gauntlet alongside Eitri. However, before they can finish their mission, the Watcher informs Gamora that's he's chosen her. Killmonger is next on the list, with the Watcher recruiting him mere moments before Shuri and Pepper Potts bust into Wakanda's throne room to take him down. Finally, as Ultron bots attack his reality, Party Thor joins the fray.
The group arrives at the same pub Captain Carter took Steve to in Episode 1, and they meet Doctor Strange Supreme before learning about their mission and being dubbed "the Guardians of the Multiverse." After Thor tries and fails to escape, the team agrees to help and begins to debate how they should go about it. It takes some deliberation, but eventually, Gamora convinces everyone they can destroy Ultron's stones using the "Infinity Crusher," a device of her creation.
Strange Supreme takes the group to an uninhabited reality to prepare for the battle, but Thor quickly alerts Ultron of their presence by conjuring lighting. However, while Ultron believes he can make quick work of the Watcher's recruits, Strange Supreme places a protection spell on everyone, granting them immunity to his attacks. Seizing the opportunity, T'Challa steals the Soul Stone right from under Ultron's nose. And as soon as the all-powerful AI notices, Strange Supreme opens a portal that brings the zombies from Episode 5, including Wanda Maximoff, to distract him.
Of course, he takes her out with ease and travels back to his original reality, where the Guardians of the Multiverse are trying to destroy the Soul Stone. However, their plan is thrown for a loop when Natasha arrives and steals the gem, not understanding how there could be other people on Earth. Captain Carter talks her down by revealing their bond in her reality, but Ultron arrives soon after, and a fight breaks out.
The group eventually pins Ultron and gets the Infinity Crusher in place, but Ultron reveals that, in each reality, the Infinity Stones are different, making her weapon useless. The AI then realizes that all he has to do is kill Strange Supreme to remove the protection spells and launches a full-frontal assault, which tosses Captain Carter and Natasha away to the location of the arrow holding Zola.
With the help of Captain Carter, Natasha hits Ultron in the eye with the arrow, allowing Zola's algorithm to enter his body and destroy it from within. Unfortunately, as soon as the team stops to take a breath, Killmonger takes the gems and tries to convince everyone they can fix their problems. They decline, of course, but before Killmonger can enact his plan, Zola resurrects Ultron's body to reclaim the Infinity Stones. Realizing the Watcher didn't want the gems destroyed, Strange Supreme traps Killmonger and Zola in a pocket dimension, ending the threat.
Strange Supreme agrees to watch over the pocket dimension as the rest of the Guardians of the Multiverse head back to their realities -- well, except for Natasha. Not wanting to return to a world with no one in it, the Watcher takes Natasha to the world Loki took over in Episode 3, giving her another chance to fight alongside the Avengers. Then, in a mid-credits scene, Captain Carter is aboard the Lumerian Star when she and Natasha stumble upon the Hydra Stomper and a mysterious figure inside it.
To see the Guardians of the Multiverse assemble, What If...? Episode 9 is streaming now on Disney+.