Malachi and Mally Trenglos live on the coast of Cornwall and depend on the resources of the sea to make a living. Malachi harvests seaweed from the ocean to sell for revenue, and as he ages, he employs the assistance of his granddaughter, Mally.

Though she is small, Mally is quite strong. The townspeople note her resolve and claim that she "[knows] nothing of fatigue." In fact, they believe that Malachi himself never gathered half the seaweed that Mally collects each day. She pours herself into the work, not caring about her supposed need to look feminine or whether her clothes reflect the appropriate ornate touches for church attendance.

When "interlopers" invade her cove and take some of the seaweed, Mally is furious. This establishes the central conflict of the story. Barry needs some of the seaweed to use as fertilizer for his family's farm, and Mally isn't willing to share the resources. Barry insists that the sea is "free to all" and that he is therefore free to utilize its resources. As they collect seaweed on a particularly bitter day in April, the two argue continue to dispute the rights to the seaweed. Mally watches as Barry slips off the edge of a rock, certain that he is going to drown. She saves his life, which completely transforms their relationship; they later share ownership of the seaweed after being married.