Dexter, having chasing Mandark in his cape (as well as Dee Dee) out of his lab, while Mandark was try to steal the all-powerful Neurotomic Protocore, was attacked by a group of robots seeking to eliminate The One who Saved the Future, yet he defeated them all. Dexter was fascinated with being the One who saved the future and so jumps into his time machine to find out.
On the first time period, He met Number 12, working for Mandark, who is his rich, successful boss. (The scariest scene from the special occurs in this time period, in which Mandark gives Number 12 a whipping.) Number 12 and the Present Day Dexter left the first time period he visited, just as Mandark snagged the Neurotomic Protocore.
On the second time period, Both Dexters laughs at Mandark's brain. Both Dexters were spotted by a museum curator who took him to Old Man Dexter, who join the two Dexters.
In the third and final time period Dexter visited, the whole world was turned broken and stupid by Mandark, the Overlord who, in the third time period, forbids science and knowledge to all except for himself. The three Dexters stumbled upon a buncha village idiots and then Action Hero Dexter who agreed to join the 3 Dexters.
Together, they build a robot to break in to Mandark's castle, where Mandark the Overlord summons his 3 selves from 3 time periods (including that Mandark in his cape!!!) to fight the 4 Dexters, but in the end, Dee Dee won by pressing the button on Mandark's machine.
The Dexters goes up in smoke, so did the Mandarks. The enraged Dexters build these group of robots which we saw in the beginning and send them to Dexter's original time period to supress her, setting the entire events in motion, which Dexter ignores and decided to eat lunch in the end.