In a dark room lit only by a single portal of daylight, a dying priest clutches a metal box. The priest was protecting the box and soon it is found out why.
The box contains a key that opens an area beneath a derelit church. The area contains a room that holds a terrifying secret.
A Vicar (Donald Pleasence) begs for help from a professor called Howard Birack (Victor Wong) and a group of physics students to investigate this room and a mysterious cylinder in the basement of the derelict Los Angeles church.
The cylinder contains a twisting, green gooey liquid.
The students attending the experiment are Walter, Susan, Wyndham, Kelly, Lisa and Catherine.
Among these people investigating this is a Metaphysican who is named in the film as Marsh (Jameson Parker).
Next to the cylinder is a book inscribed in three different languages which they try to decypher.
After reading the written passages found next to the cylinder, it is discovered that the liquid is actually Satan, the devil incarnate. The liquid itself appears to be a living organism, producing increasingly complex data that is revealed by computer decoding to include differential equations and sending the computers into data overload. One of the students decides to call it a night and is brutally murdered by a tramp (Alice Cooper) with part of a bicycle frame. Another student called Wyndham is brutally stabbed with scissors after having bugs crawl on his face and body.
The next two days develops into something sinister, small jets of liquid escape the cylinder and possess the group one at a time, causing them to attack and incapacitate the remaining students. From outside Wyndham returns back from the dead and says he has a message from Satan for the rest of the group and just after he says the words: "You're not going to like it". His body erupts with beetles emerging from his body. He loses his head and more bugs crawl out until he whole body disolves and only his suit remains.
A student called Conor tries to take his own life with a piece of stair bannister that he proceeds to jab into his throat.
They try to escape the church and they are stopped outside by possessed homeless people who are trying to kill them. Birack and the Vicar soon start to realise that Satan is actually the son of an even more dangerous and powerful force of evil, the "Anti-God",who Satan plans to bring into this world and therefore damn it for eternity.
The survivors that still remain find themselves sharing the same dream, apparently a subconcious vision that has been sent as a warning to not just the group but also mankind from the future year which is supposed to be taken in the year 1999 shows a distorted vision sequence of a shadowy figure emerging from the front of the church.
The vision and the shadowy figure shown seem to change slightly with each and every reoccurance of the dream. A voice overheard as the vision plays out each time warns the 'dreamer' that they are witnessing an actual broadcast from the future and they must alter the course of events to prevent Satan from completing his evil plan.
Eventually, the cylinder is opened and the entire gooey substance is entered orally into the body of Kelly, one of the students who becomes the ultimate incarnate of Satan: A gruesomely disfigured being, with powers of Telekinesis and regeneration, who attempts to bring the Anti-God through a dimensional portal using a powder puff mirror.
A task that the possessed Kelly fails at first because the mirror is too small to bring his father into the human world. Realising his mistake, Satan finds a larger wall mirror, and begins to pull the Anti-god's hand through it as most of the group are immobilized in fights with the other possessed members.
Catherine Danforth (Lisa Blount), who Marsh is in love with, begins to sob. She doesn't know what to do, whether to save Marsh from the possessed Conor or stop Satan's plan. She feels that seeing as she is the only one not immobilized like the others, decides to stop Satan's evil plan and tackles the possessed Kelly with both of them falling through the portal in the mirror.
The Vicar then smashes the mirror with an axe, trapping Satan, the Anti-God and Catherine on the other side of the mirror. Catherine is seen briefly on the other side of the mirror reaching desperately out to the portal before it closes, leaving her in total darkness. Immediately the possessed drop dead and the homeless people who are no longer under Satan's spell walk away from the church.
Walter sees this as an advantage and runs away into the night. The survivors are rescued, feeling relieved that the evil has been prevented and the nightmare is over. At the end, Marsh has the recurring dream again, except that this time it's a possessed version of Catherine that is now the figure emerging from the front of the church as they all visioned earlier.
He appears to awaken, and rolls over to find Catherine, gruesomely disfigured with her face covered with blood or what looks like bloody muscle instead of skin, lying in bed with him. He awakes, this time for real. Glistening with sweat, shocked and screaming, he gets up off the bed and approaches his bedroom mirror, hand outstretched. The film cuts to black just before his fingers touch the mirror, thus giving the audience the impression that the horror isn't over.