This is the third book in the Ring trilogy. In the first book, Ring, the videotape was the vendetta of Sadako, a tragic girl who could not be sated with simple revenge. In the sequel Spiral, a mutating virus displaced the tape and came to threaten the entire diversity of life.
This third book is quite different the first two books and focuses on a cancer that is threatening the human population. At the beginning of the story we neet ten-year old Kaoru Futami, a bright inquisitive child whose father Hideyuki is a scientist who had worked on the Loop project, a virtual matrix of thousands of supercomputers.
Kaoru’s father is dying of cancer and at the hospital Kaoru meets Reiko Sugiura whose young son Ryoji has cancer. Kaoru falls in love with Kaoru and travels to the deserts of New Mexico looking for answers to the origin of the cancer, motivated by his love of Reiko and to get answers for his father.
The end of the book is quite surprising and it turns out that Kaoru has actually come from the Loop. The storyline reminded me a lot of the Matrix with alternate realities and vast computer simulated worlds. Whereas the first two books in this trilogy are in the horror genre, this book lies squarely in the ream of science-fiction.
Wikipedia has a detailed summary of the plot.