U.S. Fish and Wildlife agent Venus Diamond is summoned back to her Seattle family fold to help her twelve-year-old stepbrother, Tim, who's been accused of murder. The victim is nine-year-old Pearl Pederson, who drowned while Tim, his friend Henry and the girl were playing aboard the family yacht. The further Venus digs, the more she learns that whatever happened was anything but child's play.
Venus has trouble believing Tim's story that a giant jellyfish grabbed Pearl and dragged her under - but when toxic venom is found in the young victim's body, and witnesses spot a giant sea creature roaming Elliott Bay, Venus dives into its murky depths for a closer look. Her probe leads her to the criminal elements at the heart of the mystery: enemies old and new in a network of child pornography, black market trading and the manufacture of weapons-grade biotoxins - but not before more deadly violence takes its toll.