(1999) is a landmark hour-long television special that served as the grand finale for the show's original run. Rather than a standard episode, it was the first-ever Cartoon Network original movie and marked the directorial debut of series creator Genndy Tartakovsky . The "One Who Saved the Future"

The plot kicks off when robots from the future invade Dexter's lab to destroy "the one who saved the future". Intrigued by his own eventual greatness, Dexter uses a time machine to travel forward and meet his future selves:

Together, the four Dexters engage in an epic "four-on-four" battle against various versions of Mandark. Dexter's Laboratory: Ego Trip (TV Movie 1999) - IMDb

A weak, tall adult Dexter working a miserable office job for his boss, Mandark.

A wise, elderly version who holds the final secret to defeating Mandark.

A buff, rugged version of Dexter who lives in a desolate future where Mandark has taken over the world.