Your probably know the outline version. While McCain was a POW, his beauty-queen wife Carol nearly died in a car crash. She survived, but was crippled and disfigured. They were reunited in 1973, and stayed together until 1980; one month after the divorce, he married an heiress.
The story can be read in different ways. One reading makes it pretty unremarkable: they both changed, a lot, during the years they were separated; they gave it their best shot, but couldn't make it work. He has always admitted some fault in the matter, and still picks up Carol's medical bills. She has his bumper-sticker on her car. Que sera sera; move along, nothing to see here.
The other reading makes McCain look pretty bad. He was a notorious womanizer before the marriage, and after his return home was living a high-powered public life in which his wife had no place, and adultery followed. Friends and acquaintances, including Ross Perot (who paid for a lot of Carol's medical care) are quoted to damning effect -- McCain has no character, he's cruel, etc.
Frankly, we lean toward the first and more charitable reading. It is easy to imagine, and seems to fit with what both parties have said over the years, and continue to say. But if the second and more sinister version is correct, and McCain were to win the general election, we would all be in a mess of trouble.