What If...? is an animated series that premiered on Disney+, created by A.C. Bradley. It is based on the Marvel comics and each episode has a different plot. This series explores the alternative timelines in the Multiverse, its shows these timelines with the premise that what would've happened if in the Marvel Movies some things went differently.

The narrator of th series is The Watcher who knows about all the timelines but has taken a vow of not interfering.

In the first episode, Peggy Carter becomes Captain Carter instead of Steve Rogers becoming Captain America. In the second episode T'Challa is taken by the Ravagers mistakenly while they had to take Peter Quill, T'Challa ends up becoming StarLord . T'Challa in the end also goes back to Wakanda.
The third episode sees Hank Pym, kill all the Avengers because his daughter Hope dies.
In the 4th Epsiode we see Strange Supreme, in this timeline Chritine dies and Stephen travels to learn the art of magic to revive Christine but he's told by The Ancient One that it can't be done as its an absolute point and if changed it will destroy reality in itself. His personalities are split we see his dark side gain powers by absorbing magical creatures. In the end he even absorbs his other self. In this timeline only Strange survives and he can't save Palmer.
In the 5th episode we see a zombie apocalypse. In the 6th episode we see Killmonger save Tony Stark in Afghanistan. In the end Killmonger becomes the Black Panther. He kills Tony and T'Challa. We see Thor as an only child in the seventh episode and he raises up to become quite a brat.
In the 8th episode we see Ultron win and he even manges to get all the Infinity stones and goes onto conquer many timelines and even threatens to kill The Watcher. Hence, finally in the final episode we see The Watcher break his oath and he gathers heros from the previous episodes and they defeat Ultron.
This series just shows how many possibilities Marvel still has in its Multiverse.