- "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 only saw 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."
- ―The Watcher
Ultron successfully uploads himself into a new body made out of vibranium. Armed with the Mind Stone, the rogue AI manages to kill Thor, Bruce Banner, Steve Rogers and Tony Stark before unleashing Earth's entire arsenal of nuclear weapons upon humanity, leading to their decimation. When Thanos arrives on Earth with the other five Infinity Stones, Ultron effortlessly kills him with the Mind Stone and takes the Stones for himself. Ultron's newfound omnipotence makes him aware of the greater universe.
Ultron thus expands his mission to bring his version of peace to all corners of the cosmos. Spawning a fresh army of Ultron Sentries with the Reality Stone and creating a spherical spaceship, Ultron initiates his conquest, inflicting his will on every inhabited world of the universe. Asgard, the Sovereign, Sakaar and Ego are just some of the countless planets that fall to the genocidal AI. During his Xandar invasion, Captain Marvel attempts to destroy Ultron by ramming him into the planetary core. However, she is killed when he uses the Stones to obliterate the planet. After remaking all of the universe in his image, Ultron is left as a program without a purpose. But when he notices The Watcher's presence, he learns of alternate realities within the Multiverse. Alarmed, the Watcher decides to turn his attention to that universe's last hope as the AI actively tracks him down.
Natasha Romanoff and Clint Barton are the only known survivors of the nuclear holocaust, spending the remainder of their days outrunning the Sentries. In Moscow, they infiltrate the KGB archives and attempt to find intel regarding an AI that can take on Ultron. After searching through countless numbers of files for hours, Romanoff locates the file on Arnim Zola, to the Watcher’s relief as he observes. Determining Zola as a viable combatant to Ultron, Romanoff and Barton head to a HYDRA facility in Siberia, containing a second set of databanks for Zola's mind outside Camp Lehigh.
After activating the algorithm, Romanoff and Barton coerce Zola to help them destroy Ultron from the inside. After downloading Zola into one of Hawkeye’s arrows, Black Widow lures the Sentries to the base, enabling Hawkeye to transfer Zola to one of the Sentries and allow him to invade the hive code. Despite this, Zola is unable to execute his task, as Ultron has left their universe. As the Sentries swarm the facility, Hawkeye-- having lost his will to live after the death of his family-- sacrifices himself to destroy the robot horde, buying Black Widow and the Zola sentry ample time to escape.
Simultaneously, Ultron confronts the Watcher in the Nexus of All Realities. In their fight, the Watcher teleports to numerous universes in a futile attempt to flee. Ultron gains the upper hand as he mocks the Watcher for the oath he swore preventing him from interfering before attempting to kill him. But the Watcher manages to break free and escape to another universe, where he finds Doctor Strange, who beckons him to break his oath. With Ultron's ambitions threatening the Multiverse as a whole, the Watcher reluctantly agrees and asks the sorcerer for help. Elsewhere, Ultron had taken control of the Nexus and now has all realities at his fingertips.
- Jeffrey Wright as Uatu/The Watcher
- Jeremy Renner as Clint Barton/Hawkeye
- Lake Bell as Natasha Romanoff/Black Widow
- Toby Jones as Arnim Zola
- Ross Marquand as Ultron / Ultron Sentries
- Josh Keaton as Steve Rogers
- Mick Wingert as Tony Stark/Iron Man / Tony Stark/Iron Man (recap)
- Alexandra Daniels as Carol Danvers/Captain Marvel
- Benedict Cumberbatch as Stephen Strange/Doctor Strange
- 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
- Steve Rogers/Captain America
- Bruce Banner/Hulk
- Thaddeus Ross
- Peter Quill/Star-Lord
- Drax the Destroyer
- En Dwi Gast/Grandmaster
- Ultron (recap)
- Clint Barton/Hawkeye (recap)
- Bruce Banner/Hulk (recap)
- Thor (recap)
- Natasha Romanoff/Black Widow (recap)
- Steve Rogers/Captain America (recap)
- Jimmy (recap)
- Bucky Barnes (recap)
- James Montgomery Falsworth (recap)
- Vision (recap)
- Moscow, Russia
- New York City, New York
- Seoul, South Korea
- Oslo, Norway
- Siberia, Russia
- Golden City, Wakanda
- Washington, D.C. (footage)
- White House (footage)
- Puente Antiguo, New Mexico (recap)
- Willowdale, Virginia (recap)
- Culver University (recap)
- Kunar, Afghanistan (recap)
- Camp Lehigh (mentioned)
- Ego's Planet
- Ultron Offensive
- Ultron Offensive (indirectly mentioned)
- Infiltration into the S.H.I.E.L.D. Crater Investigation Site (footage; recap)
- Battle at Culver University (footage; recap)
- Battle of New York (footage; recap)
- World War II (footage; recap)
- Kidnapping of Tony Stark (footage; recap)
- Exoskeleton Armor
- Atomic Bomb
- Hawkeye's Bow and Quiver
- Hawkeye's Prosthetic Arm
- Black Widow's Bite
- Black Widow's Batons
- Ultron Sentries
- Infinity Stones
- Starforce Uniform
- Stealth Cloak
- Red Guardian's Shield
- Iron Man Armor: Mark XLIII
- Regeneration Cradle
- Star-Lord's Helmet
- Quad Blasters
- Drax the Destroyer's Knives
- To be added
- Guardians of the Galaxy
- United States Army
- Nova Empire
- Dora Milaje
- Wakandan Royal Guard
- KGB (mentioned)
- HYDRA (mentioned)
- Winter Soldier Program (mentioned)
- S.H.I.E.L.D. (mentioned)
- Avengers (recap)
- S.H.I.E.L.D. (footage; recap)
- United States Air Force (footage; recap)
- HYDRA (footage; recap)
- United States Army (footage; recap)
- Howling Commandos (footage; recap)
|Captain Marvel Theme||Pinar Toprak|
- This is the first episode of What If...? that does not feature a closing narration from The Watcher.
- The universe depicted in this episode has several inconsistencies with the main universe:
- Black Widow is seen in a recap with other Avengers and the Regeneration Cradle, when in Avengers: Age of Ultron, she had been captured by Ultron at that time.
- During the shot where Uatu explains how Vision was created in the Sacred Timeline, Scarlet Witch and Quicksilver are notably absent, when they were present for this scene in Age of Ultron, and it was Wanda reading Ultron's thoughts through Vision's body that cued her in to Ultron's true plans.
- Ego is shown being attacked and killed during Ultron's campaign against all life in the universe, although he should have already been killed a year earlier as shown in Guardians of the Galaxy Vol. 2, which takes place before Age of Ultron.
- The Guardians of the Galaxy are shown on Sovereign presumably conducting their mission there as seen in the opening scene of Guardians of the Galaxy Vol. 2. This also occurred a year before the events of Age of Ultron and before they crossed paths with Ego.
- Thanos is seen wielding the nearly-complete Infinity Gauntlet at the time of Age of Ultron. In the main timeline, he would not begin his quest until three years after Avengers: Age of Ultron, during Avengers: Infinity War.
- The Tesseract, and with it, the Space Stone, was located on Asgard during the events of Age of Ultron.
- Despite the Power Stone being located on Xandar in the main timeline, which Thanos destroyed to acquire it off-screen in Infinity War, the planet is shown to be intact, and is, in fact, the last surviving world to be attacked in Ultron's genocidal campaign.
- Thanos is shown to be in possession of the Time Stone, which at the time of Age of Ultron, would have been in the possession of the Ancient One.
- Thanos is also shown with the Reality Stone, which at the time should be in the possession of the Collector in Knowhere.
- Although Thanos has the Soul Stone, Gamora is shown to be alive battling Ultron Sentries with the Guardians of the Galaxy on Sovereign.
- The story of the episode is similar to the What If...Age of Ultron? comic.
- The first issue had a universe where Wasp died making Hank Pym creating a more vicious Ultron. This leads into a world where Ultron successfully destroys all life on Earth. Later in the comic Ultron learns of the multiverse and invades the other universes that were shown in the comic only to be defeated by the multiversal hero team, Avengers Infinity. This is similar to this episode where Ultron is also successful in killing all life in his universe, until he learns of the multiverse. He continues his quest to kill all life across the entire multiverse until he was defeated by a multiversal hero team, the Guardians of the Multiverse.
- The scene of Hawkeye sacrificing himself to save Black Widow from the Ultron Sentries is a mirroring of a scene from Avengers: Endgame, in which Black Widow sacrificed herself on Vormir, so Hawkeye could obtain the Soul Stone.