- “We've seen this before, a universe in the final days of destruction. But this particular story... This, this one breaks my heart.”
- ―The Watcher
- “The end of the world began with one man's dream. He saw a world of peace. With the Mind Stone, Tony Stark created Ultron. But Ultron saw only one path to peace. From puppet to puppet master, Ultron required one thing. An organic body to call his own, one that fused the powers of man, machine and mind into one. In your universe, the Avengers stole the Cradle and used it to create the hero, Vision. But in this universe, Ultron got his wish. With the infinite power of the Mind Stone, Ultron began to lay waste to the planet. But the Age of Ultron was only beginning.”
- ―The Watcher
In this universe, the Avengers weren't able to steal the body Ultron was preparing for himself and, as result, they never created the hero Vision. Instead, Ultron managed to finish the body and transfer his mind to it. He gets past the protection of the scattered J.A.R.V.I.S. on the government computers and launch the nukes, ruining Earth. Increasing the misfortune, Thanos arrives two years early having all the stones except the one with Ultron. Intrigued by the knowledge of life beyond earth and the stones, Ultron quickly bisected Thanos down the middle and melts the Infinity Gauntlet for the stones. Absorbing the stones, Ultron takes an armored form resembling his old body; while recreating his sentries.
Imbued with the power of all six stones, Ultron become unstoppable. He went across the universe, going from world to world with his army, wiping out life; even the Asgardians, Nova corps, Kree, Ego and Guardians of the Galaxy stood no chance against him. Among the worlds that fell was Sakaar, with the Grandmaster watching in horror. Captain Marvel tried to stop Ultron by plunging him into a planet's core, but Ultron released a shockwave of the stones' full power, wiping out all life in the universe; bringing his vision of peace, but ultimately leaving him without a purpose.
- “The realization nearly broke the machine. With his mission complete, Ultron was now just a program without a purpose, the victor without a war sentenced to spend all of eternity... alone... basking in the boundless silence of his universe, Ultron ascended to a previously unobtainable level of consciousness. He became aware of another... he became aware of the...”
- ―The Watcher
The Watcher suddenly realizes that, to his horror, Ultron has started to become aware of not just him, but of other universes as well. He blocks his window and concludes that if Ultron got out of his universe, the Multiverse would be in danger. Nevertheless, he holds onto hope that the heroes of Earth would know what to do.
Meanwhile, Hawkeye and Black Widow searched the archives, looking for an AI named Arnim Zola. The Watcher became desperate that the multiverse is in crisis, and tries to get involve by telling Clint and Natasha of where Zola's file is. With the file found, they head to the HYDRA base in Siberia, containing a second set of databanks for Zola's mind outside Camp Lehigh. After activating the algorithm, they coerce him for help to help them destroy Ultron from the inside. With that agreement, Hawkeye uploads Zola's AI to one of the sentry, but Natasha shot both legs as a pre-caution shot in case Zola tries to betray them.
Ultron finally breaks through the dimensional barrier, confronting the Watcher. Seeing more worlds for him to kill, Ultron begins trying to destroy the Watcher. However, he is more than a match for him. As Ultron begins infecting the gap between the universes with his observation, he knocks the Watcher through several of of the universes. Ultron notes fighting is more fun and a lot less creepy on the Watcher's part. He pummels the Watcher, which each blow sending them to a different world. Ultron grabs the Watcher's head and begins burning it. The Watcher bashes the hand away and flees. Ultron returns to the gap between realities, ready to track the Watcher.
The Watcher ends up in back in the remains of the destroyed universe of Strange Supreme, where the Watcher catches his breath. A solemn Strange Supreme approaches him. The Watcher tells him that he is out of options. Strange asks if he's ready to break his oath. An annoyed Watcher asks Strange if he really must say it, to which Strange tells the Watcher he wants to hear the request. The Watcher sighs and tells Strange that he needs the sorcerer's help.
- The Watcher: "I'm out of options. That thing has left me no choice."
- Strange Supreme: "Been there. Been living the dream alone in a prison of my own making ever since. Are you ready to break your oath?"
- The Watcher: "You want me to say it?"
- Strange Supreme: "Oh yeah. Oh yeah. I wanna hear you say it."
- The Watcher: "Okay. I can't believe I'm about to say this. I see now. I need your help."
- ―The Watcher and Strange Supreme
- Jeffrey Wright as The Watcher
- Jeremy Renner as Clint Barton/Hawkeye
- Benedict Cumberbatch as Doctor Strange/Strange Supreme
- Lake Bell as Natasha Romanoff/Black Widow
- Toby Jones as Arnim Zola
- Ross Marquand as Ultron/Sub-Ultron Sentries
- Josh Keaton as Steve Rogers/Captain America
- Mick Wingert as Tony Stark/Iron Man
- Alexandra Daniels as Carol Danvers/Captain Marvel
- Terri Douglas as Additional Voices
- Robin Atkin Downes as Additional Voices
- Piotr Michael as Additional Voices
- Dave B. Mitchell as Additional Voices
- Arthur Ortiz as Additional Voices
- Ashley Peldon as Additional Voices
- Matthew Wood as Additional Voices
- Shelby Young as Additional Voices
- Despite being an episode based on Avengers: Age of Ultron, the twins Pietro and Wanda do not appear or are mentioned, including during the Watcher's initial narration, where he explains that Ultron managed to finish his new body and transfer his mind to it, while in the film the twins help the Avengers while they attempt to steal the body before Ultron can do it.
- Thanos is easily killed by Ultron, suggesting Vision could have done the same in Infinity War, had he not been injured by Corvus Glaive.
- Clint's middle name is revealed to be Francis.
- Natasha uses a shield like the Red Guardian's.
- Clint letting go of Natasha's hand and falling to his death, is a reference to Avengers: Endgame when he tries to stop her from sacrificing herself and she falls to her death.
- This is the first episode that does not feature a closing narration from the Watcher.
- This is the fifth time Tony Stark has died in the MCU, but fourth episode to feature his death.
- The location in which Ultron kills Tony, is the same one Tony visited called the NEXUS Internet Hub, from the movie.
- Just before Tony dies, dead versions of the Avengers - Captain America, Hulk and Thor - are seen just like Tony's vision of the same outcome by Scarlet Witch's magic in Avengers: Age of Ultron.
- The fact that Thanos already had the power, soul, reality, space and time stones when he met Ultron, meaning he started his quest for the Infinity Stones earlier than he did in the main timeline, as in the main universe, he didn't get them until 3 years later in 2018.
- The Guardians of the Galaxy are seen fighting on the same planet they were at the beginning of Guardians of the Galaxy Vol. 2. Given that this episode takes place in 2015, a year after the events of the movie, it means that they would have finished the mission they did in the movie, since Vol. 2 takes place a month or 2 after the first movie.
- A couple of the names on the files that Natasha finds are names of workers from Marvel Studios, including Paul Lasaine who is the production designer for this series.
- The Watcher's armour is like the one worn by a Watcher named Aron the Renegade in the comics.
- A guy in time square is the same one who wore the pineapple shirt in Vegas in the previous episode.
- Steve Rogers is seen becoming President of the United States, something that does happen in the comics.
- The Watcher on the ground looking over his shoulder to see Strange mirrors episode 4, "What If... Doctor Strange Lost His Heart Instead of His Hands?", when Strange did the same to look for the Watcher standing behind him.