Do not believe the poster and DVD cover art for this film. There is no busty female who gets bitten by a gold bug. The gold bug is real enough, but the only people who use it are some crusty old men. *shakes head at silly movie marketing*
Okay, so first we get this prologue. It tells us about an alchemist/watchmaker in the 16th century, fleeing the Spanish Inquisition (apparently, you CAN expect it *rimshot*), who was trying to create a device that would give him the key to eternal life. 400 years later, in 1932, a building in Germany collapsed, and under the rubble lay a strange man with marble skin - the alchemist who had lived all this time. He had succeeded in making the device, but though many people looked for it, it was never found. Until...
Many decades later, an old antique dealer named Jesús Gris finds it in the base of a statue. Not knowing what it is, he dusts it off and examines it. He winds the device, a gold scarab, and its spidery legs suddenly pop out and clutch his hand while a sharp needle pierces his skin and injects him with something. We later discover that there is some kind of insect within the device itself, which is producing whatever substance is being injected.
Some familiar themes start to present themselves, but with interesting variations. Jesús starts to look and younger the more he uses the device - his hair becomes thicker, his wrinkles disappear, he's more sexual - though he never actually becomes a young man (perhaps he would have if he had used the device properly or for a longer period of time). Oh yeah, and he also starts to thirst for blood, though like many essentially good people who become vampires, he doesn't take to this at all. He and his wife have a granddaughter who lives with them, and she knows her grandfather is using the device and is concerned about him - at one point she even hides the device from him inside her teddy bear.
But someone else is looking for the device, and has been looking for it many years. A rich but very sick man named Dieter de la Guardia has collected all kinds of artifacts from the alchemist, in hopes of stumbling onto the device. He eventually buys the statue where Jesús found it, though Jesús has already removed it. This leads to a confrontation, since de la Guardia has the notebook were the alchemist wrote very specific instructions on how to use it and what the cost was of using it. Jesús, unaware all this time that there were rules, has just been using it as best he could decipher on his own.
There's a rather gross scene where Jesús, who is believed to be dead at one point, wakes up in a morgue and escapes, his mouth having been sewn shut. He also finds out that his skin, which was damaged and dead-looking, can be peeled off to reveal the marble skin that we saw on the alchemist when he died.
This movie is pretty spectacular. Probably my favorite aspect is Jesús's relationship with his granddaughter, Aurora. Del Toro's movies have the best movie children. He hates Hollywood movies with silly one-liner-spewing kids, and always makes a point to show how complicated children are, as well as how unsafe it is to be a child. See The Devil's Backbone and Pan's Labyrinth for more awesomely complicated kids. He's not afraid to put kids in danger in his films. Aurora is like Jesús's little Gal Friday. She keeps his secret about the Cronos device. She's not afraid when she sees him after he's supposedly dead. She even tags along when he goes to confront de la Guardia. And it's Jesús's temptation to feed on her that convinces him to do the right thing in the end.
Fantastic movie. If you can stand a little grossness, I'd definitely recommend it (though it can be a bit hard to find).