Full story
The Watcher observes the multiverse without ever stepping in, and he presents stories from realities where one detail changed. In the first, Peggy Carter takes the super-soldier serum instead of Steve Rogers and becomes Captain Carter. Steve fights beside her in a Hydra suit of armor, the Hydra Stomper, and vanishes in an explosion. In the end, Peggy goes through a Tesseract portal and reappears seventy years later in front of Nick Fury and Clint Barton.
Each of the next episodes visits a new world. T'Challa is taken by the Ravagers instead of Peter Quill and becomes a big-hearted Star-Lord who even wins Thanos over to the good side. In another world, the future Avengers die one by one before they can assemble, and Fury learns the killer is Hank Pym, grieving his daughter, Hope. Doctor Strange loses Christine Palmer instead of his hands and, trying to save her at any cost, destroys his own universe. A quantum virus turns the heroes into zombies, and the survivors head for Wakanda with the Mind Stone.
Erik Killmonger saves Tony Stark in Afghanistan, earns his trust and uses it to become Black Panther, until Shuri and Pepper Potts see through him. Thor grows up an only child with no Loki around and becomes a party prince who turns Earth into an interplanetary rager. The tone flips in episode eight: Ultron wins. He takes Vision's body, gets all six Infinity Stones, ravages Earth and discovers the multiverse. Natasha Romanoff and Clint survive however they can, and Ultron ends up fighting the Watcher himself.
In the finale, the Watcher breaks his oath and assembles the Guardians of the Multiverse: Captain Carter, Star-Lord T'Challa, Party Thor, Killmonger, a Gamora from another world and Strange Supreme, plus the Natasha from Ultron's world. Natasha hits Ultron with an arrow carrying Arnim Zola's mind, which takes over the robot's body. Killmonger betrays the team to grab the Stones, and he and Zola fight over them.
Strange Supreme traps Killmonger and Zola in a pocket dimension and stays behind to guard them forever. Every hero goes home. The Natasha from Ultron's world goes to live in the reality where the original Natasha died. Peggy returns to her world and, on a mission with Black Widow, finds the Hydra Stomper with someone inside. That comes back later.