Full story
What If...? is an anthology: each episode shows a universe where something went differently, always narrated by the Watcher, a cosmic being who observes the multiverse without being allowed to interfere. Season 2 brings standalone stories such as Nebula joining the Nova Corps, an Ego-raised Peter Quill attacking Earth, Happy Hogan turning into a monster at Christmas, Tony Stark stranded on Sakaar, and Hela landing in ancient China alongside Xu Wenwu. It also introduces Kahhori, a young Mohawk woman who gains powers from the Tesseract and drives Spanish conquistadors off her people's land.
The thread tying the season together is Captain Carter. After facing a Steve Rogers controlled by the Red Room, Peggy Carter is taken by the Watcher to other worlds, including a universe styled after the 1600s. In the finale, she learns that Strange Supreme, the Doctor Strange who destroyed his own universe, has been abducting people from other realities to rebuild what he lost. Carter, Kahhori, and other variants team up against him. Strange Supreme ends up accepting his mistake and frees his prisoners.
Season 3, the last one, brings another batch of stories: Bruce Banner creating a giant monster fought by Avengers in mechs, Agatha Harkness in 1930s Hollywood, the Red Guardian saving the Starks from the Winter Soldier, Howard the Duck's wedding to Darcy Lewis and the birth of their daughter, Byrdie, and a Western starring Shang-Chi and Kate Bishop. Across these episodes, the Watcher steps in more and more to save lives.
That draws the attention of three other Watchers, led by the Eminence. They put the Watcher on trial for breaking his oath, and his meddling starts to crack the multiverse. Captain Carter assembles a team with Kahhori, Byrdie, and Storm to rescue him, with surprise help from Infinity Ultron, who dies buying the group time. The three Watchers merge into one giant being, and Carter throws herself at it. Her strike splits them apart and sends everyone to Strange Supreme's universe, the one place where Watchers lose their powers.
Carter vanishes in the process, leaving only her broken shield. The Watcher convinces the Eminence to watch with curiosity and respect instead of coldness. Kahhori, Byrdie, and Storm take the oath and become new Watchers by his side. The series ends with the Watcher more human than ever, grieving Carter, and with her fate unanswered.