Suburban New Jersey, Christmas Eve - 1947: A Christmas Eve experience traumatizes Harry. He catches his mother being sexually groped by Santa Claus (actually Harrys dad). The child then goes up to the attic and cuts his hand with a shard of glass from a shattered snow globe.
Thirty-three years later, an adult Harry now works in "a lousy position" at the Jolly Dreams toy factory. At home, he has taken it upon himself to become the next true Santa; he sleeps in costume, and his apartment is resplendent with Christmas toys and dcor. From the roof of his building, he uses binoculars to spy on neighborhood children to see if they have been 'bad or good' (two children are doing household chores and playing with their doll, the third child, Moss Garcia, is shown rifling through a Penthouse magazine and cutting out a nude photograph). Harry runs back home and writes Moss' name in his Bad Boys & Girls book.
On his way home, Harry peeps into the window of a local bar and sees coworker Frank Stoller, whom earlier that day had phoned-in sick and Harry had relieved on the assembly line. Harry becomes angry and rushes home, hums a Christmas tune, and breaks one of his male dollhouse figures.
The following morning Harry phones his younger brother Phil (Jeffrey DeMunn) and cancels Thanksgiving dinner.
The Company Christmas party is in full swing. Harry watches the owner of Jolly Dreams making a televised announcement that, if production increases sufficiently, the company will be able to donate toys to the disadvantaged children at Willowy Springs State Hospital. Just then, Harry is greeted by coworkers Ben, who thanks him for taking Frank's place at the warehouse, and Frank, who introduces him to newcomer-employee George.
At home, Harry realizes that people generally consider him a "schmuck" and is constantly exploited by others. He then has a nervous breakdown, and he is now convinced that he truly is the proverbial jolly old elf. Down in his basement workshop, he begins smelting toy soldiers (with swords at attention) and small axes.
Alone after-hours, Harry gets a brilliant idea. He steals Company toys, wraps them, loads them into his van, and later drops them off at the Hospital. He then leaves a bagful of dirt at bad-boy Moss Garcias doorstep, and thereafter murders three parishioners who happen to taunt him at the bottom steps of a nearby church. Coincidentally, Frank and George are also attending the midnight mass, and witness Santa's bloody crime.
Later that evening Harry manages to invade two family's homes for different reasons. After sneaking into Phil's home, he destroys his nephews' Jolly Dreams gifts, and delivers the newly-minted soldiers, as well as toys from his own workshop. At the Stoller home, he sneaks in through their basement, delivers the kids' toys, murders Frank in bed, and escapes in his van.
Christmas morning, Phil begins to suspect something is seriously wrong with his brother and argues with his wife Jackie. Their children are preoccupied with watching a television program and do not seem to mind playing with their damaged and sub-par toys.
At the closed warehouse, Harry gains entrance and activates the assembly lines, breaking even-more toys in the process. That night he drives off, and his van becomes stuck in a beautifully decorated street with plenty of lights, sending him further into a delusional state. By now his costume is dirty and dishevelled. He eventually finds himself on the wrong side of his brother's neighborhood, and a torch-bearing mob pursues him. Nevertheless, he manages to drive his van out of the sludge and arrives at his brother's house.
Phil quickly realizes that his brother is the homicidal Santa he has heard announced on the news and proceeds to choke him unconscious. He loads him into the front seat of the van whereupon Harry comes-to, cold-cocks him, and again drives off. During his escape, the oncoming mob forces him and his van off a bridge. At that very moment Harry becomes even-more delusional, thinking his vehicle is actually Santa's flying sleigh (before the accident finally kills him).