In a quiet rural California town, a couple heads to a drive-in theater, only to be butchered by an unseen assailant who uses a sword to decapitate the man and skew the woman through the neck. Investigating this dual homicide are police detectives Det. Mike Leary, John F. Goff, and Det. John Koch, Bruce Kimball. They begin by questioning the drive-in’s boorish manager, Austin Johnson, Robert E. Pearson, and the odd custodian, Germy, Douglas Gudbye. Germy mentions that a peeping tom likes to cruise the area to watch couples and lone girls, and he is instructed to try and write down the voyeur’s license plate number the next time he sees him.
That night, the killer strikes again, impaling two lovers while they are making out in their vehicle, and leaving a sword behind. To determine if the sword belongs to the missing drive-in owner, Germy is brought in to identify it. Germy states that the sword is not part of the owner’s private collection, and tells the detectives that the voyeur was at the drive-in around the time of the latest double murder, and that he managed to write down the man’s license plate number. The plate number is connected to Orville Ingleson, Norman Sheridan. The detectives visit Orville’s home. Orville denies any connection to the deaths, but when a bloody cloth is found in his car, he panics, and tries to make a run for it. Orville is caught, and claims the blood was just from a dog he accidentally ran over, which is confirmed by further analysis, forcing the police to let him go.
That evening, the detectives (one of them disguised as a woman) go to a screening at the drive-in, and spot Orville there, even though he had promised to stay away from it. After a customer who had stormed off when his girlfriend refused his advances returns to his car, he discovers that his girlfriend has been beheaded. Leary and Koch rush to Orville’s car, and find him dead from a slit throat. Austin and Germy are brought in the station for questioning, and Austin antagonizes the detectives, refusing to close the drive-in without a court order, and firing Germy.
The following evening, Leary and Koch get a call about a machete-wielding man who has just murdered two people being cornered in a warehouse, with a little girl he has taken hostage. The detectives go to the warehouse, and after a chase and stand off, shoot the man dead, learning afterward that he was a mental patient who had escaped only a few hours ago, and thus he cannot be the serial killer.
At the drive-in, Germy collects his things, and goes to the projection booth to confront Austin about which one of them gets to keep the owner’s sword collection, and about money he is owed. As soon as Germy enters the booth, the silhouette of Austin being killed with a sword is projected onto the drive-in’s screen while a Wild West movie is being featured. Leary and Koch (who want to talk to Austin) arrive just in time to see this, and break into the booth, where they find both Austin and Germy hacked to pieces and the killer gone with no trace.
The film suddenly comes to an abrupt end where an on-screen text states that other drive-ins throughout the country are now being plagued by similar bloodbaths, and that the killer’s identity is still unknown. A fake public address then announces that a psychopath is loose in the viewer’s own drive-in theater, and urges the audience not to panic, as the police are on their way.
