John Carpenter's modernized possession film depicts a bunch of scientists and grad students investigating a mysterious liquid canister locked away in the basement of an abandoned church in LA. After all kinds of scientific jargon gets tossed around, a whole lot of numbers are crunched, and a bunch of ancient text gets translated, the team finds out that the ancient mechanical cylinder houses the physical embodiment of the devil, in some kind of transcendent liquid form. Of course, the stuff leaks out of the canister, possesses the kids one by one, and eventually triggers an outbreak.
The plot gets really confusing when the physicists and priest who hired them start debating the concept of Satan coming from a negative realm of antimatter, where there is some kind of polar opposite God in wait, trying to cross over. Basically, the kids get possessed, the unpossessed ones fight to survive, Alice Cooper plays a random homeless guy keeping everyone in the church, the main host of the devil tries to reach into a mirror to pull the anti-God out into our world, and the heroine tackles them both all the way back into the mirror. The priest throws an axe at the mirror, breaking it before anything can come back out, trapping the girl, Satan's possessed body, and the anti-God all on the other side.
The heroine's boyfriend has weird dreams (which are actually broadcasts from the future) that extend the dreams everyone else was having at the church, showing that the girl who sacrificed herself to stop Satan and the anti-God eventually becomes possessed and crosses over into our world years from now to cause the apocalypse. Basically, this guy is afraid that the girl he liked might one day return from the negative world and destroy everything.