The investigators are contacted by a rancher named Eli Jonas, who lives on the shores of a lake near Walsenburg in southern Colorado. His cattle have been mysteriously mutilated and he wants to know why. The investigators drive down to his ranch at Lake Maria for the weekend.
At the ranch, they meet Jonas, a rustic man with an off-color sense of humor, and his creepy ranch hand, Joe Gibson, who wanders around with a large pick and seldom speaks. Jonas tells them that he believes theres some kind of creature inhabiting the lake which is responsible for the cattle mutilations. After examining the property, the investigators go out with Jonas onto the lake in a small boat in the evening. They hear the noises of an unidentified creature, and see motion in the water. Suddenly, Jonas bursts into flames and plunges into the water, never to be seen again. The investigators must get themselves back to shore, all the while hearing more strange noises.
The next day, the investigators feel helpless and trapped until creepy Joe brings them a wooden box which he found buried near the lake shore. Opening the box, the investigators find the journal of a priest named Andrew Cusack, written in 1847. It tells of his encounter with a monster in the lake called a lloigor, and how he imprisoned it by using a stone carved with an Elder Sign. The journal says:
...If the stone is lost a replacement must be fashioned with the cryptic elder sign and submerged in the lake. Only the Spectral Boatman must be summoned to take the holder of the stone out into the lake.... He being a phantom of some watery netherworld and a vital ingredient of whose purpose I know not. The caster of the spell must stand on the shore of this abnormal lake where I shal conceal this writing. Out over the waters in the dark of the night call forth the boatman.... And a payment must be rendered for the service of the boatman, that being at least two pounds of the flesh of an animal..... These insorcellments I have done to banish the Lloigor back to the abyss, but the gate remains hidden below the lake locked only by the Elder Sign....
Figuring that the Lloigor has once again gotten loose, the investigators plan to perform the ritual described in Father Cusacks journal. While some of them prepare a new stone with an Elder Sign, others go out on the lake, hoping to catch a fish that weighs at least two pounds. They succeed.