In Vietnam, US soldier Andy Brooks is shot by a sniper and falls to the ground. As he begins to die, he hears his mother's voice calling out, "Andy, you'll come back, you've got to, you promised." The voice becomes sinister and muffled as Andy's eyes close. Sometime later, his family receives notice of his death in combat.
Back home, Andy's father, Charles, and sister, Cathy, begin to grieve, but his mother, Christine, becomes irate and refuses to believe that Andy has died. Hours later, in the middle of the night, Andy arrives at the front door in full uniform and apparently unharmed; the family accepts the notice of his death as a clerical error and welcomes him back with joy.
Over the next few days, Andy displays strange and erratic behavior, dressing in an unusually concealing matter and spending his days sitting around the house listless and anemic. Meanwhile, local police investigate the murder of a local trucker, who was found with his throat slashed and his body drained of blood after telling diner patrons that he'd picked up a hitchhiking soldier.
Charles attempts to confront Christine about Andy's erratic behavior, which only leads to tension between the couple as Christine insists that Charles was too withholding and authoritarian a father and Charles telling Christine that she's made Andy too sensitive by smothering him. Andy continues to display unusual behavior, attacking a neighborhood boy who attempts to demonstrate his karate skills and then attacking the family dog when it tries to protect the child. At night, Andy becomes inexplicably lively and animated, wandering the town and spending time in the local cemetery. It ultimately becomes apparent that Andy has returned as some kind of vampire, and has been draining people's blood in order to reinvigorate himself, injecting it into his own decaying body with syringes.
On a double date at the drive-in with his high school sweetheart Joanne along with his sister and best friend, Andy begins to die from a lack of blood and attacks first Joanne, then his friend. They are both killed during the attack. The other drive-in patrons witness the last attack and panic. This causes Andy to flee before he can infuse himself with his friend's blood. The police pursue Andy, the chase ultimately ending at the graveyard where Andy has been spending his free time. There they discover Andy, finally dead and reduced to a skeleton, lying in a shallow grave that he has been progressively digging for himself beneath a crudely fashioned tombstone. Christine, looking at her son's body, gravely tells the police "Some boys never come home."