He was the mercenary who was hired by Sheldon J. Plankton to kill SpongeBob SquarePants and Patrick Star. The reason being that Plankton stole King Neptune's crown and tricked Neptune into believing the crime was committed by Eugene H. Krabs, so Neptune threatened to execute Krabs (via burning by fire) unless the crown was retrieved within a period of six days; Plankton, obviously, wanted Krabs to die from a painful and wrongful execution, and upon learning SpongeBob and Patrick's journey to get the crown back, he sends Dennis after them.
SpongeBob and Patrick encounter Dennis for the first time after reaching the end of the Deep Dark Trench. Dennis was about to kill them with his "big" spiky boot, only to be stepped on by the Cyclops' (a scuba diver) "bigger boot". In the climax of the film, SpongeBob and Patrick hitch a ride on the backside of David Hasselhoff, who speedily swims through the ocean in order to send them back to Bikini Bottom. During the middle of their swim, SpongeBob and Patrick see the boot belonging to the same diver who squished Dennis. Dennis then detaches from the boot and onto Hasselhoff, revealing that he is still alive. He tries to kill SpongeBob and Patrick, but fails yet again. He gets knocked off by a boat Hasselhoff swam under, never to be seen again by the viewers. It is unknown what became of him after this, but it was most likely he just fell back into the water or he could have died.
Dennis appears as the boss in 2 of the 4 boss levels in the Movie game; "Name's Dennis" and "Dennis Strikes Back!"
In "Name's Dennis", he places SpongeBob on a stick and attempts to throw projectiles and use smaller enemies to defeat Patrick. Patrick counters and defeats him, and as Dennis is about to counterattack them, he is stepped on by the cyclops.
He, deformed physically, later appears in "Dennis Strikes Back!"; this time, SpongeBob fights him and he must use the bowling ball (and if possible the reflective macho strike attack) to defeat Dennis. After being defeated, he is about to approach Spongebob, though he still falls off into the ocean after being hit by a heading, his fate unknown.