I've seen a lot of bad movies in my time, but I've never seen quite so spectacular a mess as this.
There are three parallel stories being told. In the present day, Tommy Creo (Hugh Jackman) is a cancer researcher fighting desperately to find a cure that will save his wife Izzy (Rachel Weisz). She's more accepting of her impending death than he is; knowing that she will be unable to finish the novel she's writing (in longhand, using a fountain pen, no less), she asks him to write the last chapter.
That novel is our second story. It's set during the Inquisition, when a conquistador named Tomas (Jackman again) is sent to "New Spain" by Isabella (Weisz again) in search of the Mayans' legendary Tree of Life, which she believes will allow her to end the Inquisition's campaign against the crown.
The third story takes place (I think, though it's never specifically stated) in the distant future, and features a bald-headed man (Jackman yet again) who may or may not be the same guy we're seeing in the other two stories; he's floating through space in a sort of cosmic snowglobe with a dying tree that looks an awful lot like the Mayan Tree of Life. He's haunted by visions of Izzy/Isabella, and they are all headed for the dying star that was, in Mayan mythology, the location of the underworld.
The Fountain is, to be sure, visually spectacular, but we haven't seen this sort of deeply symbolic pretentious visual imagery since 2001: A Space Odyssey, and that at least made some sort of sense for most of the way; this one is a perplexing mess throughout, and I defy anyone to offer a coherent explanation of how the three stories are or aren't connected. I would certainly not recommend that anyone pay to see this in a theater, but when it hits cable or DVD, it might be worth a look just for the mind-numbing, jaw-dropping, train-wreck-y horror of it all.