The Fantastic Four: First Steps

Set in a separate universe with a retro-futurist 1960s aesthetic, not on Earth-616. Reed Richards, Sue Storm, Johnny Storm, and Ben Grimm have been public heroes for four years following a cosmic-radiation accident.

The Silver Surfer — here Shalla-Bal, who became Galactus's herald to spare her homeworld Zenn-La and then spent centuries selecting other worlds for consumption — arrives to announce Earth's consumption.

Whether a Silver Surfer exists on Earth-616 has never been addressed; this one is native to the retro universe.

Galactus is a cosmic entity who consumes planets. He isn't evil in a moral sense — he describes an insatiable hunger he cannot refuse, and he predates the current universe. He wants Franklin Richards because Franklin's cosmic potential could take on that hunger permanently: a successor who would free him.

He offers to spare Earth in exchange for the baby. They refuse. Sue gives birth to Franklin during the escape.

The team attempts to teleport Earth out of Galactus's reach; it fails. Galactus takes Franklin. Sue expends everything she has to push Galactus into a portal, and dies.

Franklin — days or weeks old — reaches out, touches her, and she revives. No explanation is offered, which is the point: he is an infant with power nobody understands, including his parents.

Mid-credits, four years later — Franklin is roughly four — Sue leaves him alone for a moment and returns to find a man in a green cloak holding a metal mask standing over him. Victor von Doom has arrived. This is Doom's first on-screen MCU appearance, silent, and it is the single most load-bearing setup for December.

The film never shows the crossing to Earth-616.

The Thunderbolts* post-credits — an extradimensional craft marked with a 4 arriving on Earth-616 — is the arrival, and Doomsday presumably covers the mechanism.

Franklin's own reality-altering ability is the most-cited fan explanation. Marvel has said this film flows directly into Doomsday.