- "Thor and Loki, a brotherhood so strong and pivotal, it would change the fate of a universe. Their childhood taught Thor many lessons. But in another universe, instead of raising the Jotunheim prince, Loki, as his own son, Odin returned him to his people. Without his trickster brother to keep things, let's say, lively, Thor grew into a very different prince."
- ―The Watcher
Instead of adopting Loki, Odin returns him to the Frost Giants, leaving Thor to grow up as his only child. As he became of age, Thor began to travel among the cosmos as the self-dubbed "party prince". In modern times, as Odin enters the Odinsleep, Frigga assigns her son to study up on the Nine Realms in preparation to become King of Asgard. Although she disallows Thor from partying, he gathers Sif and the Warriors Three to travel to Earth and perform the contrary, as Heimdall barely pays attention to the humans.
In New Mexico, Jane Foster and Darcy Lewis track Thor's arrival on Earth. They contact S.H.I.E.L.D. to inform them of an alien invasion, believing that it was the same force that decimated Alpha Star two years prior. Meanwhile, Thor and his crew land in Las Vegas and move the crowd into partying. As other-worldly lifeforms join in, Foster and Lewis also arrive and find Thor. Foster talks to the Norse God of Thunder about Alpha Star, and they begin to fall for each other. While the party goes on, Howard the Duck asks Lewis out only to get engaged soon after. S.H.I.E.L.D. Director Nick Fury attempts to reach Thor, but is accidentally knocked into a coma by Korg, leading Maria Hill to be named Acting Director.
The next morning, Foster and Lewis are summoned by Brock Rumlow and Hill to the S.H.I.E.L.D. Helicarrier to have them help eliminate the alien threat. Phil Coulson reveals that the party atmosphere has begun spreading to other corners of the world, Thor himself having left for France. As Foster objects to any sort of S.H.I.E.L.D. intervention, Rumlow brings forth Fury's pager, which Hill uses to summon Captain Marvel.
As Loki and his fellow Frost Giants arrive to join his "brother from another mother" in France, Captain Marvel arrives and demands Thor to leave Earth. When he refuses, the two engage in a globe-spanning fight, which ends back in France in Thor's favor. He leaves Captain Marvel to be humiliated as a "party pooper". Back at the Helicarrier, Hill scolds Captain Marvel for her defeat, but Captain Marvel explains if she exert her full power she will blow a crater in this planet which will cause lots of fatalities. After a romantic phone call with Thor, Foster attempts to stop S.H.I.E.L.D.'s plan to drop a nuclear bomb on the Asgardian, but she and Lewis are dismissed by Hill.
The party begins to grow destructive as various landmarks are either damaged or vandalized. Foster calls Thor again, but it is picked up by Loki, who accidentally breaks the phone. Lewis gives Foster the idea to contact Frigga through Heimdall, which successfully gets her summoned to Asgard. She tells Frigga that her son was on Earth. Meanwhile, in Sydney, Thor is abducted by Captain Marvel and taken to Siberia. As S.H.I.E.L.D. prepares to drop the nuke there, Frigga appears in front of Thor. The Party Prince nervously tells her that he was studying as part of an exchange group, but Frigga decides to find out for herself. This turn of events greatly amuses Captain Marvel, as well as Hill, who decides to drop their attack.
Fearful of Frigga's impending arrival, Thor decides to end the party, which angers his audience. However, when Thor announces his mother's imminent arrival to them, they become so alarmed by this news that they agree to help clean up after themselves. Once she lands on Earth, Frigga finds Thor and his party crew studying, with Captain Marvel lending him a tablet loaded with information on human civilizations. As the Asgardians prepare to leave, Thor retrieves Mjølnir, but he finds it covered with party material.
Back in New Mexico, Thor and Foster arrange for a date. As Thor leaves, The Watcher forwards a saying of the two "living happily ever after". However, much to his surprise and Thor's shock, a portal appears in front of Thor, in which Ultron and his army of Ultron Sentries emerge, who had assumed a vibranium body and now brandishes the six Infinity Stones.
- Jeffrey Wright as Uatu/The Watcher
- Chris Hemsworth as Thor
- Natalie Portman as Jane Foster
- Tom Hiddleston as Loki
- Kat Dennings as Darcy Lewis
- Samuel L. Jackson as Nick Fury
- Jeff Goldblum as En Dwi Gast/Grandmaster
- Cobie Smulders as Maria Hill
- Clark Gregg as Phil Coulson
- Frank Grillo as Brock Rumlow
- Taika Waititi as Korg
- Karen Gillan as Nebula
- Jaimie Alexander as Sif
- Seth Green as Howard the Duck
- Alexandra Daniels as Carol Danvers/Captain Marvel
- Rachel House as Topaz
- Josette Eales as Frigga
- David Chen as Hogun
- Fred Tatasciore as Volstagg / Drax / Additional Voices
- Max Mittelman as Fandral / Additional Voices
- Clancy Brown as Surtur
- Kimberly Bailey as Additional Voices
- David Jordan Chen as Additional Voices
- June Christopher as Additional Voices
- Terri Douglas as Additional Voices
- Robin Atkin Downes as Additional Voices
- Michael Ralph as Additional Voices
- Kaitlyn Robrock as Additional Voices
- Kari Wahlgren as Additional Voices
- Matthew Wood as Additional Voices
- Shelby Young as Additional Voices
- Frigga's Sister
- Yondu Udonta
- Rocket Raccoon
- Sovereign Chambermaid
- Thor (recap)
- Loki (recap)
- Odin (recap)
- Nine Realms
- Puente Antiguo, New Mexico
- Las Vegas, Nevada
- Paris, France
- Amesbury, England
- New York City, New York
- Black Hills, South Dakota
- Pisa, Italy
- Florence, Italy
- Bosnia and Herzegovina
- Sydney, Australia
- Siberia, Russia
- St. Louis, Missouri
- North Dakota (mentioned)
- Mojave Desert (mentioned)
- South America (map)
- Greenland (map)
- Canada (mentioned)
- Mongolia (mentioned)
- Pennsylvania (mentioned)
- Hawaii (mentioned)
- Jotunheim (recap)
- Asgardian Armor
- Horned Helmet
- Bifrost Bridge
- Fake Infinity Gauntlet
- Infinity Stones
- Melt Stick
- Transmitter Pager
- Yaka Arrow Controller
- Gamma Radiation
- Starforce Uniform
- Ultron Sentries
- Asgardian Communicator
- Winged Helmet (recap)
- Casket of Ancient Winters (recap)
- Atomic Bomb (mentioned)
- Frost Giants
- Fire Demons
- Love Bots
- Polar Bears
- Gary (mentioned)
- Flerken (mentioned)
- Goose (mentioned)
- Cats (mentioned)
- Ducks (mentioned)
- Horses (mentioned)
- Rabbits (mentioned)
- Unicorns (mentioned)
- Eagles (depictions)
- Cows (depictions)
- Asgardian Royal Family
- Warriors Three
- Sakaaran Guards
- PBS (mentioned)
- National Public Radio (mentioned)
- Erik Selvig
- Darcy Lewis' Father
- Loki's Mother
- George Washington (statue)
- Thomas Jefferson (statue)
- Theodore Roosevelt (statue)
- Abraham Lincoln (statue)
- Jeff Spicoli
- Top Gun
|Themes from Thor: Ragnarok||Mark Mothersbaugh||
|Themes from Thor||Patrick Doyle||
|How Bout U (Instrumental)||Chick Norris||
|William Tell Overture||Extreme Music||
|French Flavors||APM Music||
|Captain Marvel Theme||Pinar Toprak||
|Aloha Malihini||Extreme Music||
|Paris Swing A||APM Music||
- This is the first episode of What If...? with no character deaths.
- The universe depicted in this episode has several differences with the main universe:
- In the opening recap, which shows what happened in the original universe, Loki uses the Casket of Ancient Winters against Thor as a child. However, as a Frost Giant, touching the Casket would turn Loki back into his original blue appearance, and thus should have alerted Loki to his difference as a child.
- The Fake Infinity Gauntlet, unlike the main universe version, is left-handed and has all the "Infinity Stones" with the right colors and in the place that Thanos put them in Avengers: Infinity War.
- Although Mjølnir should not have Odin's worthy enchantment, Captain Marvel can't get up when Thor puts the hammer on top of her.
- Loki was abandoned as a baby in this universe, but with Odin returning him to Laufey rather than raising him as his own. However, adult Loki is tall as all other Frost Giants, when in the original universe Odin said that he was small for a Frost Giant.
- The way in which Darcy Lewis says "They're here" when Thor, Sif, Volstagg, Fandral and Hogun arrive on Earth is a reference to a famous line spoken by the character Carol Anne Freeling (Heather O'Rourke) in the 1982 horror film Poltergeist. It was previously spoken by Quake in the Agents of S.H.I.E.L.D. episode The Force of Gravity.
- The man who marries Darcy Lewis and Howard the Duck is an impersonator of Elvis Presley, which is common in Las Vegas. Las Vegas is known for its marriages due to the minimal costs and ease of acquiring a licence to do so. The Bible that he holds is embroidered with the words "Love Me Tender", which is the name of one of Presley's most famous songs.
- A.C. Bradley referred to the lighthearted natured episode as a way to "be silly and have some fun, and kind of take a breath myself" following the depressing nature of the preceding episodes. Bradley went on to say that she was "very surprised and supported that Kevin [Feige] and Bradley [Winderbaum] were like, 'Let's do it.'" She cited 80s and 90s party movies and romantic comedies, as well as The Hangover, as inspirations for the episode.
- Loki refers to Thor as his "Brother from another mother" due to a reference to Tom Hiddleston's saying the same about Chris Hemsworth.
- What If... Thor Were an Only Child? on Wikipedia
- What If... Thor Were an Only Child? on Marvel Database