The Fountain is about two people who like each other a lot.
One of them, Izzi, is dying of brain cancer; the other, Tommy, is really sad about that.
As she is dying, Izzi writes the fictional account of Tomas and Queen Isabel.
Isabel asks Tomas, the conquistador, “Will you deliver Spain from bondage?” Tomas replies, “Upon my honor and my life.” Isabel says, “Then you shall take this ring to remind you of your promise. You shall wear it when you find Eden. And, when you return, I shall be your Eve. Together we will live forever.”
Tomas eventually encounters the man who guards Eden with a flaming sword and kills him. Tomas tries to keep his promise to Isabel by putting on the ring. Instead, he dies when he is transformed into flora. Eden, along with the dead souls of Isabel and Tomas, is then transported to Xibalba. As Izzi explains to Tommy (in reality), that “nebula wrapped around a dying star” was, for the Mayans, “the place the dead souls go to be reborn.” Izzi tells Tommy that her book begins in Spain but ends in Xibalba.
So, the dead soul of Tomas waits in dilapidated Eden for the dying star he’s drifting around to explode. The result of this explosion should be the rebirth of Tomas and Isabel.