Oscar Madison was a man with the world at his fingertips. Newly freed from a failed and miserable marriage, he was a born-again bachelor enjoying his new-found freedom and open to all the possibilities life now had to offer. But what he didn't count on was that he would cross paths with someone he thought was a friend, who would invade his life and turn the two of them into the oddest couple imaginable...
That supposed friend was one Felix Ungar, an introverted and repressed loner who had managed to destroy his own marriage through his relentlessly obsessive and compulsive behavior. Driven to the brink of insanity, the unstable Ungar makes a manipulatively half-hearted attempt to commit suicide as a way to illicit sympathy from his friends. And when that happens, Oscar is there, the perfect prey to walk right into the trap.
The unsuspecting Madison invites Ungar to move in with him. Little does he know that his carefree bachelor life is about to come to an abrupt end. For Felix's need to control knows no bounds. And Oscar's casual, spontaneous way of living sets him over the edge again, bringing out out the same side of him that caused his wife to flee and nearly drove him to suicide.
Taking on almost the role of an obsessive wife, Felix begins to slowly wrap himself around every aspect of Oscar's life. Just as Madison begins to realize the grave error he has made, he also realizes that it is too late. Before long, Felix has become inescapable, draining Oscar's life of all its vitality, sucking it into his bottomless vortex of madness as he attempts to impose his uncompromising sense of order.
Simple things like trips to the coffee shop, bowling, and poker night, things that bachelor roommates should easily be able to enjoy, become nothing short of nightmares, with Felix's deranged and self-sabotaging behavior smacking Oscar in the face at every turn. Even Oscar's long-time friends begin to back away from Felix's disturbing behavior, giving Felix more and more free reign to dominate Oscar's life.
"We're all out of cornflakes. F.U." writes Felix in a particular sinister note left on Oscar's pillow one morning. Does the F.U. merely stand for "Felix Ungar"? There's no way to know for sure.
The final move in their intricate psychological game occurs when Oscar makes the foolish attempt to mend fences with Felix by inviting the Pigeon sisters to their apartment for what he assumes will be a night of bachelorly carousal. But it's clear that he is still unaware of the depths to which Ungar's sickness runs.
Making sure that Madison is occupied, Felix twists the evening to his own personal agenda, manipulating the sisters into pitying him, abandoning any chance of amorous activities in favor of some kind of demented, parent-child dynamic. Enraged at this last betrayal, Oscar at last comes to his senses and demands that Felix exit his apartment, and his life.
And yet once again proving to be too much of a match for Madison's naivete, Ungar manages to once again overpower his former roommate with overwhelming guilt. Wracked with regret over what he has done to the supposedly alone Felix, Oscar tracks him down and begs him to come back. It is only then that he discovers the truth, and the final endgame in this fractured odyssey: Felix Ungar has moved in with the unsuspecting Pidgeon sisters. And his obsession is about to be born anew...