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.
During the journey, Dennis encounters the gas station that SpongeBob and Patrick had gone past earlier. When Floyd and Lloyd begin insulting Dennis by asking him "Does that hat take ten gallons?". Dennis does not like this at all and rips off their mouths before moving on. Later, as SpongeBob and Patrick are being chased by the frogfish, Dennis stops at the thug tug. He blows a bubble and finds a image of Patrick and SpongeBob inside to track them. He is then confronted by Victor whom begins to tell him "All bubble blowing babies will be beaten senseless by-". Dennis also did not like this at all and simply punched Victor, sending him flying to the bar. Dennis then drove away and moved on. When he arrived at the skeleton field, he began laughing upon crushing a skull. Sometime later, he arrived at the other side of the trench and waited to ambush them on the other side.
SpongeBob and Patrick encounter Dennis for the first time after reaching the end of the Deep Dark Trench. Dennis explains to them that he is a hitman hired to exterminate them. SpongeBob and Patrick laugh him off, believing that they are invincible with the seaweed mustaches Mindy gave them earlier. Dennis then ripped them off and revealed to them that they were fake. He then showed them his real mustache, causing Patrick to wonder if he was a mermaid.
Having enough chatting, Dennis prepares 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 walked up to SpongeBob and Patrick and Patrick suggested that they fight back against him.
Dennis quickly swatted him aside and went after SpongeBob and eventually grabbed him after a chase around Hasselhoff. SpongeBob tried to bride Dennis with Gooby Goober money and with bubbles, which damaged Dennis's eyes. Irritated, Dennis prepared to finish them but is knocked out of Hasselhoff by a passing boat, presumably killing him as he is never seen again.
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 gets hit by a boat.