A Satanic offshoot of the Catholic church has been operating since the 1500s, led by a renegade priest named Thomas Seaton (William Beckwith). The members can earn immortality if they sacrifice a blood relation every 13 years, and soon it will be time for George Parkman (Max Jacobs) to renew his bond with Lucifer. Parkman's intended victim is his virginal granddaughter Alexandra (Christine Moore), though her fiance Bill (Tim Gail) is suspicious of the intentions of the old man and his lecherous friend Father Seaton.

Oblivious to the danger, Alexandra rejects Bill's concern as jealousy. After her mother dies under suspicious circumstances, Alexandra moves in with her grandfather and falls further under the evil priest's spell, never suspecting that she's being prepared for a Winter Solstice sacrifice to Satan. However, the Catholic church is aware of the sinister cult's plans and a young nun has infiltrated their ranks in an effort to smash their blasphemies once and for all.