Natica and Impani are teenaged Colonial Scouts--they transport via wormholes to fantastic new worlds and report their findings. The girls were best friends until the day a terrible accident took the life of an innocent colonist. Now Natica is consumed by guilt, and she hates Impani for not understanding. She quits the Scouts and returns to her water-world home. The reunion with her family is not what she expects, however. Her twin brother is missing--and Impani has followed her home.
Impani is enchanted with the sculpted coral buildings and under-lit waterways of the ocean world. She decides to take her time trying to talk sense into Natica and treat her mission as a vacation.
The two girls and Natica's stowaway younger brother (who is both help and hindrance) set off on a perilous adventure as they search for Natica's twin. A bomb demolishes their sailboat, and when they make out a police report, the police station explodes. They are caught in a wild speedboat chase through the canals of the floating city, and are held hostage on a pirate ship where Natica finds romance and betrayal with an old boyfriend. They instigate a near riot that results in the capture of a wanted criminal.
With each new danger, Natica regains confidence. She realizes Impani is correct--she must accept her mistakes and move on. When she finds her twin brother, he tells her he plans to have gills implanted and live in a city beneath the ocean. Anguished, he admits he ran away because he feels responsible for the accidental death of a young neighbor boy. Natica learns it is not enough to accept your mistatkes--you must forgive yourself for making them. She convinces her brother to go home and leaves with Impani to rejoin the Scouts.