An architect and his wife are flying from London to L.A. with an altar from an ancient abbey secured in the plane's cargo hold. Also aboard the flight are Buddy Ebsen as a pushy millionaire, William Shatner as a drunken, cynical ex-priest, Tammy Grimes as a person with mental problems, and Chuck Connors as the lantern-jawed pilot. Crew and passengers come into jeopardy when an invisible demon escapes from the altar, and threatens the plane in an effort to destroy the architect's wife.