The investigators notice the following ad in the morning paper:

"WANTED: Daring, experienced person or people to investigate difficult problem. May be dangerous. Call Hank at 420-4272 days or 421-1032 nights."

After responding to the ad, investigators meet with farmers Hank, Jake, Tony, Cynthia and Mary Jean regarding the death of their friend Johnny O’Dell. They suspect the involvement of someone called Dr. Menghin, who they claim is affiliated with some kind of cult. They say that Johnny’s body was found next to a horrible jack-in-the-box toy, which they believe was meant as some kind of warning.

After an appointment with Menghin, the investigators search his office and discover incriminating evidence pointing to cult activity surrounding the Sumerian god Marduk, along with diagrams of a high-technology device created by Dr. Menghin for contacting the entity. The next day they learn that Dr. Menghin has been arrested for theft of plutonium.

Following an urgent phone call from Mary Jean Roshmeyer, one of the group of farmers, the investigators drive to her Arvada farm. After a long time searching, the investigators discover Mary Jean’s dead body underneath a mattress, along with a child’s jack-in-the-box with a hideous, bloody doll inside. Clues suggest that the "farmers" who hired them in the first place are themselves cultists, opposed to Menghin’s group.

At a local amusement park, Elitch Gardens, the investigators find Menghin’s high-tech transmitter, which is using the park’s Ferris Wheel as an antenna. They disarm it, and remove the crucial part, a magical object called the Eye of R’as Al Ghethi. Later, they find another of the unpleasant jack-in-the-box toys on the hood of their car, which they take to be a warning to discontinue their investigation or risk death.

In a final showdown in the basement of a Denver home, the investigators confront the farmers, and are themselves confronted by Monty Harbison, Menghin’s scientific assistant and leader of Menghin’s cult. It becomes clear that the farmers are in fact members of a cult which is opposed to Menghin’s, and that the death threats were coming from them all along. They were using the investigators in an attempt to infiltrate Menghin’s group and recover the Eye of R’as Al Ghethi, which had originally belonged to them. All of the farmers are killed by supernatural forces, and the investigators can see their bodies covering the floor of an adjacent room. Harbison demands the return of the Eye of R’as Al Ghethi, but the investigators pull a clever switcheroo using an object of identical size and shape, sealed in an envelope. Harbison discovers the trick, however, and kills all of the investigators.

Alternate Ending
Just as he’s about to shoot the investigators, Harbison is distracted by some creepy noise from the room where the dead bodies of the farmers are located, and the investigators seize the opportunity to disarm, and then to kill him. When they enter the room where they saw dead bodies just moments previously, they find that all the bodies have mysteriously vanished: the room has no other exits.