Steven Harmon was junior high school student, who's archenemy was Winston. He plotted to get back at Winston, this involved dressing like a clown at a carnival. But before he his plan could unfold, evil clowns kidnapped Winston and Heather (Steven's date). The clowns enter a portal (disguised as a mirror). Steve followed them into the portal and his molecule got stretched across 3,741 dimensions. He ended up in the realm of the Scientist Supreme of Dimension X (who kinda looks like Groucho Marx), who helped him master his new form (made of unstable molecules). The SSD thinks the reason why Steve was transformed was because Steve was "slightly out of sync" while going through the portal. Steve (as Slapstick0) stopped the evil clowns and their leader, Overlord, from brainwashing captive humans into being his slaves.
Slapstick has had several other enemies: a "copycat of the Punisher", Overkill, who wanted kill Slap for being different. He stopped Neutron Bum, a homeless man able to create explosions, by giving him coffee and knocking him out when he wasn't looking. He helped the New Warriors battled Dr. Yesterday (mainly to impress the ladies).
Slapstick's identity was made public during the Civil War. He is seen going to Camp Hammond and stopping the Hulk and working on several missions.
Powers and Abilities
As Slapstick, Steve Harmon is indestructible, any damage is quickly shaken off. He has super durability, agility and reflexes. He also has unlimited malleability ( in the manner of a Loony Tunes character). A specific frequency of energy can turn him into a puddle and back. He can get superhuman strength when electrocuted. He has alien technology most notably pockets links into another dimension allowing him to store virtually anything in there. Steve is a skill prankster.
Speedball (Slapstick's best friend) described Slap as "The hero who doesn't fight crime but rather plays cruel tricks on it."