What If...? season 3's fifth episode opens on a world with an army of drones called the Iron Federation marching through the streets. We find Riri Williams, a.k.a. Ironheart, heading to a motel as The Watcher explains that she fails her mission in every version of this reality.
Meeting with Sharon Carter, Riri acquires an analogue part from an Easy Bake oven that's an essential part of whatever she's building. The Power Broker betrays her, though, and the Iron Federation closes in - many of them are revealed to be holograms - as Ironheart makes her escape in a makeshift suit.
However, it's then that White Vision attacks and, just as things look bleak for the hero, she's rescued by Wong, Okoye, Valkyrie, and Ying Nan. This is no normal Earth; it's the remnants of our planet in a reality where the Emergence happened years before the Eternals stopped it.
With Earth left in literal pieces by Tiamut's birth, Quentin Beck/Mysterio made his move and took over Stark Industries, creating an army he now controls to keep the survivors under his thumb. However, he's dying and wants to put an end to those standing up against him.
Riri is tasked with creating a device that can kill White Vision and the Alliance heroes buy her some time while she finishes it. Their army is wiped out by the villainous synthezoid but Riri succeeds and comes up with a plan to infiltrate his base and destroy the nanites powering his illusions. It's a good idea, though The Watcher is despondent over the fact he'll have to watch her die again.
Merging with White Vision's tech, Ironheart becomes a human synthezoid and things look good until the suit fails and she can no longer see through Beck's illusions. Her teammates hold off the Iron Federation as she closes in on him; unfortunately, it was all part of Mysterio's plan and she's walked straight into a prison cell.
He reveals that uploading the nanites into himself means he can control illusions with his mind; alas, it's also killing him so he now intends to merge with White Vision and live forever as Master of Illusions. Riri fights back but it's revealed that she never left the asteroid field; Wong, Okoye, Valkyrie, and Ying Nan are all dead and everything since White Vision was defeated has been an illusion.
The Watcher prepares himself to see Riri fall again, but can't bear it and encourages her to fight back. She beats Mysterio and projects the Avengers logo into the sky to give the survivors hope. Beck dies and the day is saved.
However, three more Watchers have their eye on Uatu: The Incarnate (D. C. Douglas), The Executioner (Darin De Paul), and The Eminence (Jason Isaac). It seems he's now interfered one too many times...
In a Universe where The Eternals never stopped The Emergence, the birth of an incubating Celestial shatters the Earth. Civilization endures on the rocky remnants of our planet, where Quentin Beck leads an authoritarian regime until freedom fighters recruit Riri Williams on a deadly mission to take him down.
The cast for episode 5 includes Jeffrey Wright, Dominique Thorne, Alejandro Saab, Emily VanCamp, Tessa Thompson, and Michelle Wong. The episode is directed by Stephan Franck, with a story by Bryan Andrews, Matthew Chauncey, and Ryan Little, and a teleplay by Matthew Chauncey and Ryan Little.