Nine-year-old twins Viola and Orion have a home like no other. They live in the Ice Spire Library, a glowing tower of frozen light in a city built deep inside an ice bubble on the distant, blue planet of Triton. Their days are spent sliding through crystal tunnels, drinking spicy geyser-cocoa, and listening to the whispers of ancient story-crystals. But their perfect, icy world is about to shatter. It starts with a rumble. Then, a terrifying "Great Geyser Quake" rips through their city, shaking the ice-spires and cracking the floors. In the chaos, their Grandfather Atlas’s most treasured possession is lost: a mysterious old locket. This is no ordinary locket. It's a key. The quake has silenced the Singing Stones, the ancient, humming crystals that provide all the city's light and heat. Now, a dangerous cold is creeping in, and the city of Cryopolis is freezing.

With their grandfather heartbroken and their home dying, Viola and Orion know they are the only ones who can find the locket. Their quest will force them to leave their safe, bubble city and brave the dangers of Triton's forbidden surface. Their adventure will take them across the slushy, bubbling Methane Sea, where they meet giant, gentle Glow-Squids that light up the gloom. They’ll discover the frosty wreck of an ancient starship, the Star-Seeker, and team up with a scattered, three-eyed mapmaker. To find the key, they must face glittering Diamond Caves, escape a shadowy monster, and uncover a secret that lies deep below the frozen crust—a secret about their family and the true purpose of their planet. Viola and Orion have always been close, but now they must learn to think and act as one. The locket is the key, but their bond might be the only thing powerful enough to find it and reawaken the heart of Triton. Can two children solve a riddle older than their city? Or will the lights of Cryopolis go out forever?