In a future where an adult Gwen Tennyson is president and the world is being taken over by the alien horde known as the Xerge, a disillusioned Ben 10000 is called out of retirement to recruit his 10-year old version from the past, only together can the battle-worn adult and the impulsive child save a future under siege.

When Ben meets a young Generator Rex on the run from a hostile Providence, he must work through a series of misunderstandings, and defeat the dark wizard Hex in order to save the world from a malfunctioning Omnitrix infecting the world's human population with alien DNA.

When the Tennysons are attacked by a villainous, dimension-hopping Omni-alien known only as Alien X, they soon meet a parallel universe version of Max who opens the door to hosting multiple Bens and Gwens, forming an alien hero army to combat this threat.