John Lefsburg, a small town detective, has lost his will to live. Demoted to mundane duties, he is assigned to transport a homeless man, Simon Applewhite, to a psychiatric ward in the nearby city of Madison. Simon claims that aliens are invading the earth from the moon. His mission is to kill media mogul Frank Coen, who Simon believes is an alien staging an invasion to destroy the human race. Through a series of mishaps, John's and Simon's lives become intertwined and both must help each other in order to reconcile their troubles.