A priest invites Professor Howard Birack and his students to join him in the basement of an abandoned Los Angeles church. He requires their assistance in investigating a mysterious cylinder containing a swirling green liquid. Among those present is Brian Marsh, a student in theoretical physics.
They decipher the text found next to the cylinder which describes the liquid as the corporeal embodiment of Satan. The liquid appears sentient, and broadcasts increasingly complex streams of data. The academics use a computer to analyze the data, and find that it includes differential equations. Over a period of two days, small jets of liquid escape from the cylinder. Members of the group exposed to the liquid become possessed by the entity, which uses them against the others. Attempts to escape from the building are thwarted by a mass of possessed street people who surround it and barricade the doors from the outside. Two members of the research group are killed.
Birack and the priest theorize that Satan is actually the offspring of an even more powerful force of evil, the "Anti-God", who is bound to the realm of anti-matter. The survivors find themselves sharing a recurring dream (apparently a tachyon transmission sent as a warning from the future year "one-nine-nine-nine") showing a shadowy figure emerging from the front of the church. The hazy transmission changes slightly with each occurrence of the dream, revealing progressively more detail. The narration of the transmission each time instructs the dreamer that they are witnessing an actual broadcast from the future, and they must prevent this possible outcome.
Eventually, the cylinder opens and the remaining liquid is absorbed into the body of student Kelly, who transforms into the physical vessel of Satan: a gruesomely disfigured being, with powers of telekinesis and regeneration. Satan attempts to summon the Anti-God through a dimensional portal using a mirror, but the mirror is too small and the effort fails.
While the rest of the team is occupied fighting the possessed, Kelly finds a larger wall mirror and draws the Anti-God's hand through it. Brian's lover and fellow physicist, Catherine Danforth, is the only one free to act: she tackles Kelly, causing both of them to fall through the portal. The priest shatters the mirror, trapping Kelly, the Anti-God, and Catherine in the other realm. Catherine is seen briefly on the other side of the mirror reaching out to the portal before it closes. Immediately, the possessed die, the street people wander away, and the survivors (Brian, Birack, The Priest, and Walter) are rescued.
Brian has the recurring dream again, except that now Catherine, apparently possessed, is the figure emerging from the church. Brian awakens and finds Catherine, gruesomely disfigured, lying in bed with him. This is still part of his dream, however, and he awakens screaming. Rising, he approaches his bedroom mirror, hand outstretched.
Who is Brian's lover?
Catherine Danfort