Satan is preparing his legions to break out of Hell and overwhelm the Earth. He has been planning this for millennia, and intends to win the Battle-of-Armageddon with a preemptive strike. Satan's preparations for the battle were finalized during the end of the 20th century, when Hell had claimed the souls of all the Nazi rocket scientists, as well as the inventors of the A-bomb, napalm, germ warfare, etc.
The hero is a video artist with a weekly cable show on Public Access. One morning he is awoken by the sounds of construction coming from the parking lot next to his building. Massive equipment is being moved in and the lot fenced off. Helicopters circle overhead. Instinctively he begins to video the proceedings.
He learns that a tunnel is being dug straight down, 20 feet wide and deeper than any tunnel has gone beneath the city. Signs claim it is part of the city's water project.
The hero's girlfriend notices there are lights on at odd hours in the temporary office from which the excavation is directed. She becomes suspicious of the project, and devises a plan, wherein they lower a small video camera from a fishing pole to peer into the rear window of the office. Their camera discovers that it turns into Satan's War Room by night.
Beelezebub speaks, "I congratulate you Mephistopheles, your plan has worked out beautifully so far. You offered to get that billionaire elected mayor if he agreed to let us build an entrance to Hell in the middle of his city, and he went for it. We actually have the permits we need to excavate this tunnel down to the Gates of Hell."
The hero airs this footage on his weekly cable show. Most viewers assume this is a put on, but he slowly gains a following among a variety of religious groups as Satan's battle plans unfold to an apocalyptic conclusion.
After many plot twists (the President wants to drop the H-bomb on Manhattan to stop Satan), all hell breaks loose on Earth and our hero and his girlfriend have front row seats. Four flying horses lead the charge out of Hell: one red, one red, one red and one pale. Numerous demons, devils, gargoyles, improbable beasts, mythological creatures, dragons, ghosts, and amorphous blobs of ectoplasm continue streaming out. (The President's bomber crashes before reaching Manhattan, the crew haunted by demons.)
The nightly newscasters reporting on the battle all have horns, and announce, "The war of liberation is succeeding! The television transmitters have been liberated, and free speech has been restored: you can now watch triple XXX movies on prime time network television. All drug laws have been repealed in the territories under control of the liberators. If you're over 21 you will be able to buy any drug you want, in any pharmacy, without a prescription. Gambling, prostitution, and lewd behavior are also legalized by Prince Satan's 'Proclamation of Liberation from Jehovah.' You do not need to resist the liberators. They have come to set you free."
Unless they've read the book, viewers will have to wait for parts 2 & 3 to learn how the battle ends.