I love it. Why not have it that the Church have long had a secret library full of forbidden texts deep underneath a city somewhere (the Vatican maybe? Too obvious perhaps?); most notable among them is this Delomelanicon, the book written by the Devil himself. Indy discovers proof that this fabled library exists but he's framed for murder before he can reveal this by people who want the secret kept: framing him to discredit his discovery. Their plan, a secret society of conspirators in various high places in society, is to obtain the book and travel to the Ninth Gate. Indy follows the trail and discovers the plan, but they find him and force him to break into the library, battle the guards and steal the book for them! They tell him that they're actually a band of radical priests, and have teamed up with an Industrialist (who has plenty of robot mining machines to dig to the Gate) and a religious nut with his own private army to wage war on the Devil, kill him and free the world from evil. Indy of course thinks that this is A Very Bad Idea.
At the end of the film they get to the Gate (beneath the Bermuda Triangle? Perhaps you have to fly into it with the book and are moved a second into the future, to of sync with the rest of the world: this is where the Ninth Gate is hidden), and against Indy's efforts they open it: however the villains turn against each other- one doublecrosses the others and reveals that his plan is to free the Devil! The army is killed as they enter the Gate, and then the giant mining machines emerge back through it; with glowing red eyes!
Demonically possessed robots! Just then, after searching for Indy all through the film, Mutt and his GI buddies turn up... and all hell breaks loose!