WARNING: Contains nothing but spoilers
In the middle of the 22nd century, mankind has reached a point in its technological advances to enable colonization of the far reaches of the universe. Armed with intelligent "Exponential Thermostellar Bombs", the scout ship DARK STAR and its crew have been in space alone for twenty years on a mission to destroy "unstable planets" which might threaten future colonization. Meanwhile, the ship's crew, consisting of Lt. Doolittle, Sgt. Pinback, Boiler and Talby (their Commander, Powell, has died and exists only via cryogenic supports) perform their jobs in a state of abject boredom as the tedium of their task has driven the crew up the wall.
Sgt. Pinback has adopted a ship's mascot in the form of a mischievous alien "beachball with claws" that refuses to stay put in the food locker, forcing Pinback to chase it all over the ship (O'Bannon later adapted this kernel of an idea into Alien). Lt. Doolittle dreams of surfing back in Malibu. Boiler spends his time using a laser rifle for target practice. The navigator, Talby, has become reclusive and spends all his time in the ship's dome. The computer has become dysfunctional, with parts of the ship burnt out and others simply blown up.
Due to damage suffered in an asteroid storm, thermostellar Bomb #20 thinks it has been ordered to detonate while still in the ship's bomb bay. The other crew members attempt to talk the bomb out of blowing up ("'Wouldn't you consider waiting around awhile for another course of action?' 'No.'"). Lt. Doolittle revives Commander Powell who advises them to teach the bomb the rudiments of phenomenology, resulting in a memorable philosophical conversation between Doolittle and the bomb. Bomb #20 retreats to the bomb bay for contemplation, and disaster seems to have been averted. Pinback addresses the bomb over the intercom to finally disarm it.
The bomb misinterprets Doolittle's phenomonology and believes itself to be God. He explodes, killing Pinback and Boiler instantly, Commander Powell is fired off into space encased in a large block of ice, Talby drifts off into the Phoenix Asteroids to die and circle the universe, and Doolittle surfs down to the unstable planet on a piece of debris to burn up in the atmosphere.