Roz is back, and things are peaceful on the island. Roz's son returns with his flock, and a mate, too, named Glimmer. Then, an injured seal washes ashore, and tells the inhabitants of a terrible poison in the ocean. Roz initially reassures everyone that they're safe, but soon the poison reaches the shores of the peaceful island, and Roz decides she's going to deal with the terrible problem.
She travels far (her adventures in the previous book have given her the ability to immerse herself in water with no worries) and meets whales, and other creatures, including the Ancient Shark, who tells her that a huge mining station is releasing the poison as it mines an underwater mountain.
Roz manages to talk to the humans on board, and come to an agreement, even as the Ancient Shark is rallying creatures to attack and destroy the station.
There are some tense moments, and some good lessons, as readers are taught how ocean currents can move good and bad things all over the planet, and that negotiation is a better way to handle problems.
Roz has some great moments with her beloved son Brightbill, and, Roz is a grandmother! How sweet!