ZüM is a White Martian and one of eight-such individuals who comprise the supervillain team known as the Hyperclan. Disguised as alleged "heroes" from another world, the Hyperclan sought to turn public opinion against Earth's superhero populace, in particular, the JLA.
ZüM assumed the guise of a super-speedster, which was just one of the inherent powers available to him. ZüM and his teammates Armek and Zenturion fought against rookie Green Lantern Kyle Rayner and the Flash in the Gobi Desert. Flash and ZüM raced across Beijing and it appeared as if he were actually faster than the Flash. Although he did not derive power from the same Speed Force as Wally did, ZüM was able to execute strategic super-speed maneuvers such as vibrating through brick walls, accelerating the molecular speed of the wall, turning the individual fragments into mini-grenades. He also confounded the Flash's perceptions by generating after images of himself, modulating his frequency so that they appeared to exist in multiple locations simultaneously. The Flash ultimately caught up with ZüM in South Dakota. By pushing himself to near light speed velocity, the Flash was able to reach ZüM's destination before him and ambush him. Harnessing the infinite mass that light speed travel affords him, the Flash punched ZüM, sending his body hurtling across the planet. Because of this, ZüM was not present when the rest of the Hyperclan captured the Justice League and brought them to Z'onn Z'orr.
ZüM regrouped with Armek and followed him back to their headquarters, but by the time they arrived, the JLA had already escaped and had learned about the Hyperclan's Martian heritage. This time, ZüM traded blows against Aquaman, mistakenly believing him to be the weakest link on the team. Aquaman used his telepathy to affect ZüM's basil ganglia (inherited from a species marine ancestors) to give him a seizure, ultimately incapacitating him.
When the rest of the Hyperclan were defeated, the Martian Manhunter used his powers to wipe their minds and memories, forcing them to assume human forms. Providing them with new identities, he then sent them out to live as productive members of the human race.