Responding to a call from his friend Buck Cowan, Jack Russell goes to Buck's beach house. Waiting for him there are Buck, Jack's sister Lissa, their friend Topaz, Raymond Coker and Brother Voodoo. Coker explains how his aunt and uncle were murdered by a zuvembie and he needs their help. The group next discuss the visions they've recently separately experienced, visions of the supernatural trinity the Three Who Are All.
Then the house is invaded from every side by zuvembies. With Brother Voodoo and Topaz using their powers while the rest wield makeshift weapons, they fight off the creatures, although a few manage to knock Coker out and carry him away. Finally the zuvembies are defeated and they dissipate into shimmering light. While the group look for Coker, the full moon rises and Jack transforms, but the change is reversed when Brother Voodoo sends the spirit of his brother Daniel into Jack.
Brother Voodoo, Jack and Topaz then leave for Haiti, where they hope to find the reasons for the zuvembie attacks, and possibly Coker. Already in Haiti is LAPD Lt. Victor Northrup on a 'working vacation', looking into the zuvembie attacks while searching for Coker. On a map, he circles a place named Devil's Grotto, which is also the destination of Jack and his companions.
Brother Voodoo's private plane lands some distance from the Devil's Grotto, and he, Jack and Topaz begin the rest of the trek on foot. Before long, they are attacked by real zuvembies (unlike the constructs of mystic light they fought earlier). They fight back, and Topaz is knocked unconscious and about to be killed by one of the creatures. Brother Voodoo recalls Daniel's spirit from Jack to possess the would-be killer, and in doing do, Jack transforms. Between Brother Voodoo's powers and the Werewolf's fierce slashing, the zuvembies are subdued, and Topaz comes to. Also, Jack realizes he now has a measure of control over his bestial alter ego.
They finally arrive at Devil's Grotto, finding a deep pit crawling with demonic beasts. They take a tunnel pathway to the bottom that avoids the creatures. When they reach the end, they see Coker in the lupine form he had been cured of, bound and supported in the midair by tendrils of red light. As false zuvembies emerge from the stone walls, Dr Glitternight reveals himself to be the mastermind behind these events.