Basically, the surviving member of the monarchy that Doom deposed *who had a laser cyborg eye or something* led a revolt that ended with Doom being overthrown, temporarily of course. *I think he hijacked the Fantastic Four into helping him regain power since Latveria was better off under him*.
Somewhere in this, Kristoff's parents were killed, and Doom adopted the boy. Because that's what you have to do when you're king. Later, after Doom was... lost in space or believed dead or something, a bunch of Doombots took it on themselves to download Doom's brainwaves into Kristoff, who went crazy, believed he was Doom trapped in a child's body, and tried to destroy Reed Richads (of course!)
Doom later came back, and the two had a war for control of Latveria, and at the climax of this *PSYCHE!* Turned out the non-Kristoff Doom was a Doombot, and the real Doom came back from space, restored Kristoff's mind, though apparently still leaving him with Doom's knowledge, and shut down the errant Doombot.