Returning to Earth in his space cruiser hoping for a vacation, action hero Duke Nukem is shot down over Los Angeles by unknown attackers. As he sends out a distress signal, he learns that aliens are invading the city and have mutated the local police. With his plans ruined, Duke ejects and vows to do whatever it takes to stop the invasion, fighting his way through a besieged city and beyond as he uncovers the scope of the alien threat.