Life for Roz and the animals on their island is perfect: wild, natural, and harmonious. Her son Brightbill has even found a mate! But then, one day an injured seal washes ashore and warns of dangerous, cloudy waters that are flowing toward the island—and soon the animals are forced inland to fight over dwindling resources.

Roz calms and organizes the animals, but the poison tide takes a terrible toll on the island. And when the robot discovers that her new body is waterproof, she marches into the waves and sets out across the ocean, determined to stop the poison tide.
 
During her undersea journey, Roz encounters amazing geological formations and incredible creatures, and she sees the devastation caused by the toxic waters. Creatures she meets along the way direct her to a mysterious character known as the Ancient Shark, who explains that the poison tide is caused by a huge station that floats on the waves; a station operated by humans and by robots. To stop them, the Ancient Shark is prepared to attack with an army of sea creatures. However, Roz hopes to find a peaceful solution, instead. Can the wild robot save the ocean and her island and everything she loves?

This thrilling third installment of the Wild Robot series takes readers on a new adventure under the ocean and to the frigid northern waters where Roz may have to make the ultimate sacrifice.