Follow thirteen-year-old Boyd Under and his older sister Maggie as they enter their grandfather's secret underwater city. Located at the bottom of the Pacific Ocean, Sans Terra is a technological marvel. Within its many domes, citizens enjoy a surprisingly normal city life of parks, shops, restaurants, schools, and apartments. However, life goes from typical to tense when several people inexplicably fall ill. It is up to Boyd, Maggie, and friends to uncover the source of the illness. Will solving this mystery unite friends or turn friends into enemies? Dive in to find out.