Dial Code Santa Claus

Thomas de Frémont, a child prodigy obsessed with tinkering and action films, lives in a secluded and high-tech castle with his widowed mother, Julie, his diabetic and partially blind grandfather, Papy, and his pet dog, J.R. On Christmas Eve, Thomas uses the Minitel to try and communicate with Santa Claus and unknowingly makes contact with a derelict who is using a public Minitel terminal. The deranged man claims to be Santa and attempts to get Thomas to divulge his address; before their connection is severed, the vagrant learns Thomas's mother manages a nearby Printemps.
While Thomas sets up a security system to record or capture Santa, the vagabond gets a job as a Santa at the Printemps but is fired from it after Julie witnesses him slap a child who accused him of not being the real Santa. The vagrant subsequently steals a Santa suit, paints his hair and beard white, and hitches a ride to Julie's home in the back of a delivery van, the driver of which he kills upon reaching the de Frémont residence. The man then murders Julie's groundskeeper and her chef, breaks into her home through the chimney, and stabs J.R. to death in front of Thomas, who is convinced the intruder is an enraged Santa. What follows is a game of cat and mouse as Thomas uses his security system and booby traps, as well as an arsenal of makeshift weaponry, to defend his enfeebled grandfather and combat the trespasser, who has cut the telephone lines and trashed the only car; the man at one point catches Thomas but then lets him go while declaring, "I win. You lose. Now I hide and you seek. Okay?"
Julie, concerned over her calls home not getting through, phones the police, who send an officer to the castle to check on Thomas and Papy. The vagrant murders the policeman and recaptures Thomas, but the boy is saved when his grandfather shoots his assailant with the dead officer's gun, with Julie arriving home seconds later to find a stunned Thomas standing over the killer's body, stammering, "It's my fault, Mom. I wanted to see Santa Claus."