Full story
In 1964, on an alternate retro-futuristic Earth called Earth-828, the world celebrates four years since Reed Richards, Sue Storm, Johnny Storm, and Ben Grimm came back from space with powers. Cosmic rays turned them into Mister Fantastic, the Invisible Woman, the Human Torch, and the Thing. They became celebrities, fought villains, and changed the planet. Reed's inventions pushed technology forward, and Sue's Future Foundation helped disarm the world. When Reed and Sue announce they're having a baby, everyone wonders whether the child will be born with powers.
Months later, the Silver Surfer appears in the skies over New York. She warns that Earth has been marked to be devoured by Galactus, a cosmic being who eats planets. Reed studies worlds that were already destroyed and confirms the threat. The four head into space on a faster-than-light ship, following the Surfer's trail, and arrive in time to watch Galactus consume a planet.
Galactus captures the team and explains that his hunger never ends. He senses that Sue's baby carries enormous cosmic power and makes an offer: he'll spare Earth in exchange for the child. He even forces Sue into labor. The family refuses, escapes by slingshotting around a neutron star, and Sue gives birth to Franklin on the way home. Back on Earth, the news becomes a scandal. Plenty of people want to hand the baby over, and Sue has to win the public back by saying they won't abandon humanity or their son.
Reed comes up with a huge plan: teleportation bridges all over the world to move Earth out of Galactus's reach, with Sue uniting the nations through the Future Foundation. The Surfer starts tearing the bridges down. Johnny figures out who she is: Shalla-Bal, a scientist from the planet Zenn-La who became Galactus's herald to save her own world and who carries guilt for the planets she helped destroy. The final plan is to use Franklin as bait and lure Galactus to the last bridge, in Times Square.
Sue uses every bit of her power to push Galactus with a force field while Reed rescues Franklin. Johnny tries to sacrifice himself, but Shalla-Bal stops him and drags Galactus through the portal, sealing it behind them both. Sue dies of exhaustion and Franklin brings her back. The movie closes with the world celebrating the team's fifth anniversary. In the post-credits scene, four years later, Franklin gets a visit from a mysterious man in a green cloak holding a metal mask, and that comes back later.