The third and final book in Percy's innovative and acclaimed Comet Cycle, The Sky Vault follows the aftermath of an aeroplane that goes missing over Fairbanks, Alaska, in the wake of the comet, and a teenager's search for answers about his father's final moments aboard the flight.

The comet, Cain, came from beyond our solar system, its debris containing elements unknown. It brought a powerful new metal to the once-declining Midwest, alien fungus to the forests of the Pacific Northwest, and now, in the isolated region of Fairbanks, Alaska, the skies shift and stretch as an interstellar dust cloud seeds itself in the atmosphere. The National Weather Service dismisses the anomaly. Then, an hour outside of Fairbanks, a plane shudders its way through pulpy, swirling, bruise-shaped clouds, lit with sudden cracks of lightning. The sky opens. And the plane vanishes.

Theo Jenson is 15 years old when his father inexplicably disappears along with the plane, and the month-long search for him ends with a funeral procession and the beginnings of acceptance. Despite the tragedy, Theo's popularity in school skyrockets, while his best friend, Wheezy, remains on the outskirts of any real social circle. Their friendship will be put to the test when other Fairbanks citizens begin to vanish, sucked up by a funnel cloud that extends like an elephant's trunk, and chased down by a mist that solidifies into the shape of a man.

Most comets, it's generally believed, are made of a combination of rock and ice. But the fact that so many of them blink out of existence—seen one day, gone the next—makes some scientists believe they are cored with something known as "mirror matter", another term for the dark matter we know exists in the universe because of its gravitational force. The effects of Cain's dark matter are still not entirely known, but its impact on Earth might be more treacherous, or sublime, than could ever be imagined.