On the vast, sunlit walls of the world's greatest monolith, two veteran climbers unwittingly ascend into a vertical underworld. In a place where obsession kills, they quickly fall prey to past loves, old demons, and ghostly revenge.

Once upon a time, thirty-five years ago, Hugh and Lewis were Yosemite legends. El Capitan was their holy grail, and their destiny seemed written on its big walls. Here they met the women who married, then left them. Hugh's wife mysteriously disappeared into the desert. Lewis's wife is divorcing him. Now, the old friends reunite to climb El Cap one last time and make a fresh start.

But "the Captain" seems cursed this time around. Even before the aging "wall rats" leave the ground, disaster strikes three young women high on a neighboring route. Hugh discovers one body in the forest and spies a second dangling a half mile overhead. He and Lewis launch their ascent, desperate to leave the terrible omens behind.

But there is no escape. Doubts haunt them, the elements plague them, and an eerie reptilian woman invades their sleep. Nothing comes free up here. Every vertical inch, the men must battle the stone, their aging bodies, fear, and the memories of great loves lost.

They doggedly climb on, only to be drawn into a deadly rescue attempt by Augustine, a search-and-rescue expert. Like a man possessed, he is hell-bent on saving his lover, one of the doomed women. Hugh ties in to the rope with him and climbs through a gauntlet of fire, ice, and deep gravity to the wreckage of the fall.

There, he and Augustine find a survivor, starved, half-mad, and holding a corpse. The battered young woman raves that El Cap requires one final sacrifice. On this tiny island in the sky, the rescuers become the victims, hounded by some ruthless spirit and caught between the golden summit and terminal velocity.