Angel, a Basque priest and professor of theology, confesses to another priest that he intends to commit as much evil as he can. The other priest is shocked until Angel whispers his reasoning. When the other priest agrees to help him, a large cross falls and crushes him. Angel goes into downtown Madrid, committing various sins and crimes such as taking money from a homeless man, pushing a street performer over, and telling a dying man to go to Hell and stealing his wallet. He goes to a record store and meets Jose Maria, a self-described Satanist and heavy metal fan, who gives him a tape of the most "evil" band he can think of. Jose puts Angel up in his mother's boarding house where he lives with his mother, grandfather, and Mina, a woman he is attracted to that helps his mother with the business. Angel then attempts to steal a book from a book store written by occult TV show host Professor Cavan, who he believes can tell him how to sell his soul to Satan. He tries to explain to the store manager his theory: a secret code in the Bible says that the Antichrist will be born at midnight on Christmas Eve, and he intends to sell his soul before the night is out to get into the birth ceremony and kill the Antichrist. The store manager does not believe him and he flees. Angel and Jose team up to kidnap and tie up Cavan, who they intimidate into telling them how to complete a ritual in which Angel can sell his soul to the Devil. The ritual requires the blood of a virgin, so Angel first tries to get it from Cavan's girlfriend Susana, but after knocking her out by mistake, Cavan admits that she is not a virgin. Angel then goes back to the boarding house, drugs Mina, and draws some of her blood, but Jose's mother sees him and tries to kill him with a shotgun. Angel escapes with some of the blood, but accidentally kills Jose's mother in the process. Angel, Jose, and Cavan are able to burn a piece of paper, take LSD, and complete the ritual, which Cavan believes - like most of his "occult" practices" - is made up. However, a goat appears in the room and stands on its hind legs in front of Angel, then disappears. Angel then finds a message in the burnt scraps of paper in which the devil taunts him, not fooled by Angel's actions. Angel, Jose, and Cavan flee the apartment when the police arrive on suspicion of Cavan's kidnapping. Angel and Jose frantically drive around town looking for a sign to point Angel in the right direction. Angel first tries to get the information on the birth of the Antichrist out of a man giving a speech about the predictions of Nostradamus, resulting in a chase in which three men dressed as the Wise Men are shot by police by mistake. He then tries to find the information at a heavy metal show, but is beaten up by the attendees. Cavan goes on television and asks Angel to call him, telling him that he's realized Angel was right and he knows where the birth of the Antichrist will be. Cavan points out that the devil uses his own markings, similar yet opposite to the Christian cross, in order to mock God, and points them toward a pair of buildings in the same shape of the devil's mark. The three men engage in a fight with a gang that has holed up there and has been seen throughout the film murdering homeless people and spray painting"limpia Madrid" everywhere. Their leader is revealed as the devil when he throws Jose off the building to his death. The gang beats Cavan badly, but Angel gets hold of a gun and kills the gang, also killing a baby who he realizes is the Antichrist. He then shoots the devil as well. Some time later, another actor takes over Cavan's show, and Angel and Cavan become homeless drifters; while Cavan constantly complains that they will never be able to tell anyone how they saved the world, Angel simply misses Jose, but has accepted the events as the duo's fate.
In a retrospective review of the film for Popoptiq, reviewer Ricky D. wrote, "Delirious, demented and diabolically funny, The Day of the Beast is essential viewing." In a retrospective review for Daily Grindhouse, reviewer Johnny Donaldson wrote, "THE DAY OF THE BEAST may not exactly be a traditional Christmas movie, even by the standards of the horror genre, but it’s a perfect one for those who want to thumb their noses at the “Christ” part of Christmas."
