Pearl is a young harbor seal, struggling to understand the secrets of a changing ocean. She is also trying to understand her mother, the Black Pearl, one of the most respected and feared members of the seal community.

The Black Pearl is hard on her daughter but tells Pearl that she must grow stronger and faster, readying herself for danger to come. "We are the guardians of our own future," her mother says. Slowly, Pearl gains independence, learning about the dangers that lurk in her ocean environment, including thinning kelp forests, great white sharks, and deadly orca.

When the Black Pearl is injured, she tells her daughter Pearl that she -with her strength and speed-is the only one with any hope of protecting the other seals. By fighting to survive the ocean's dangers, Pearl grows braver, just like her mother. Balancing the ancient ways of the seal folk with the challenges posed by the modern world, Pearl comes to a deep understanding of the sacrifices it will take for her and her community to survive and thrive.