A group of twelve people on a camping tour of the Rio Grande decide to spend the night in an old ghost town, and an unseen killer begins to dispatch them one by one. On the first night at the stroke of midnight, three of the group are killed in rapid succession. Allen (Alvy Moore) is found hanged; his friends Rod (Bobby Diamond) and John (Hank Worden) are both hacked by a cleaver.
In the morning, the nine survivors try to leave, but find their three rubber rafts slashed apart by someone (or something) forcing them to spend another night at the ghost town. During the day, two youths on motor dirt bikes arrive and one of the guides, named Jerry (John Nowak), leaves with one of them to get help from a nearby ranch which is over 30 miles away.
At nightfall, Bob (Pepper Martin) takes over as de facto leader of the group and has them set up traps to try to trap the killer, but the unseen killer seems to evade them every time leaving no evidence, not even footprints. Soon, the unseen killer strikes again, killing Andy (Bob Macgonigal) by striking him in the face with an ax and decapitating Bob with a scythe. The unseen killer murders one of the dirt-bike youths by blowing him through a door, and leaves Stan (Ethan Wayne) and the overweight, slow-witted Lou (Joe Allaine) badly injured.
At the stroke of midnight, a mysterious horse-drawn stagecoach arrives in the ghost town, being driven by a mysterious cowboy who introduces himself as Charlie Winters (Woody Strode). Charlie tells the group that he has been hunting the killer for over 40 years since the 1890s and also claims that the culprit is the ghost of an old sea captain who drove people out of town years ago. The rest of the survivors are wary about trusting Charlie, but soon realize that he may be their only hope of survival.
When Jerry is found dead and Charlie wanders off with no explanation, Rudy (Joseph Alvarado) takes over as leader of the group and takes the survivors to barricade themselves in a wood shed as the killer tries to break in. Just when Lou is pulled out of the shed and is about to be killed, Charlie reappears and shoots the killer (revealed here to indeed be an invisible force), which then drops the scythe. Charlie then rides away into the night on his black horse. Minutes later, a ranch owner and his wife arrive on the scene in a pickup truck to greet the relieved survivors.
Book ending the film is an appearance of a porcelain trinket of a chief on the mantel of an old house, supposed to resemble the sea captain and cook whose ghost may have been responsible for the killings.