The full story
Five years after the zombie outbreak of the What If...? timeline wiped out most of Earth's heroes, survivors Kamala Khan, Kate Bishop, Riri Williams, and the AI F.R.I.D.A.Y. find a working transmitter and set out for the North Institute in Ohio to activate it. On the way, a fight between Ikaris and a zombified Carol Danvers costs Kate her life and gets Riri bitten; Riri and F.R.I.D.A.Y. both sacrifice themselves to save Kamala, who is pulled to safety by Blade Knight — Blade, fused with the power of the moon god Khonshu. At the Institute, the group meets fellow survivors Melina Vostokoff, Alexei Shostakov, and Yelena Belova, and works out that reaching the Nova Corps means getting a signal into space. A horde led by a zombified Okoye and steered by Wanda Maximoff overruns their defenses; Alexei kills a zombified Steve Rogers, Blade Knight kills a zombified Ava Starr, and Melina sacrifices herself so the others can escape.
A parallel group — Shang-Chi, Katy Chen, and FBI agent Jimmy Woo — was saved early on by Shang-Chi's father Xu Wenwu and the Ten Rings; Wenwu gave Shang-Chi the Ten Rings to contain his own infection and died stopping the outbreak from spreading further. The two survivor groups meet at Baron Zemo's stronghold, the Raft, where Zemo tries to trade Kamala's group to infected Talokanil in exchange for passage — but a zombified Namor attacks the Raft outright, killing several survivors before Kamala's group brings him down. Yelena stays behind to launch the escape pod and is killed doing it; the rest reach New Asgard, where Valkyrie is talked into letting them in.
New Asgard turns out to be fed by a self-styled "Queen of Sokovia" — Wanda, claiming to have retaken control — whose food is secretly made from zombie remains, infecting the Asgardians and Alexei. Zemo and Jimmy Woo are killed in the chaos, and Thor holds Wanda off long enough for the group to launch into space and reach the Nova Corps, who respond by quarantining Earth and ordering them to turn back or be destroyed.
The Nova Corps open fire anyway. Kamar-Taj sorcerers rescue the group and reveal that Bruce Banner absorbed the energy of the Infinity Stones — destroyed years earlier when a dying T'Challa pushed a zombified Thanos into Wakanda's vibranium core rather than let him complete the Snap — and became the "Infinity Hulk" to keep the planet from being destroyed outright. Wanda arrives with a horde to claim that power for herself; a zombified Thor kills Blade Knight and several of the Kamar-Taj sorcerers, and the horde overwhelms nearly everyone else, including Shang-Chi, Katy Chen, and Valkyrie. Hulk kills zombie Thor but is left vulnerable, and Wanda seizes the Infinity Stones' energy. Kamala, out of options, helps her remake the world.
In the aftermath, Kamala wakes up reunited with Riri and Kate as if nothing happened — until reality glitches around them, and Riri reveals she's hacked into whatever this is. Nothing about the ending is what it looks like.