On Earth-828 in 1964, the world celebrates the fourth anniversary of astronauts Reed Richards, Sue Storm, Ben Grimm, and Johnny Storm becoming the superhero team known as the Fantastic Four after they gained superhuman abilities from exposure to cosmic rays during a space mission. Since then, the team have become celebrities and fought supervillains, while Reed's inventions have progressed technology and Sue's diplomacy via the Future Foundation has led to demilitarization and peace for much of the world. When Reed and Sue reveal that they are expecting a child, the world prepares for the new arrival and questions whether the child will also have superpowers.

Months later, the Silver Surfer comes to Earth and declares that it has been marked for destruction by Galactus, a planet-devouring cosmic being. Reed studies the disappearance of other planets and verifies this claim, and the team decides to return to space to stop Galactus from coming to Earth. They track the Silver Surfer's energy signature and, using faster-than-light (FTL) travel, arrive at a new planet just as it is destroyed by Galactus's ship. The team is captured and brought to Galactus, who reveals that he has an insatiable hunger that has driven him to consume planets for eons. Galactus senses that Reed and Sue's unborn child has an immense cosmic power and could take on his hunger, freeing himself from it. He offers to spare Earth in exchange for the child, and induces Sue to go into labor. The team refuses the offer and escapes from the ship. Using the gravity from a neutron star, which destroys their FTL system, the team delays the chasing Silver Surfer and propel themselves back to Earth. Sue gives birth to a boy, Franklin, on the way.