Robert Downey Jr.'s Doctor Doom is after Franklin Richards for a major reason, and the new Fantastic Four member should be key to Avengers: Doomsday.

How Franklin Richards, the power cosmic and Doctor Doom's appearance in "The Fantastic Four: First Steps" credit scene affects "Avengers: Doomsday."

In Marvel Comics, Doom’s fixation on Franklin is legendary. He views the boy’s near-limitless powers as the key to creating and ruling Battleworld, a twisted reality of his own making as seen in Secret Wars.