William G. is a divorced book seller who is estranged from his two daughters. Wandering into the London Zoo aquarium he is drawn to a set of green turtles and begins to learn about them. At the same time, popular children's author Neara H. is similarly drawn to the green turtles. Independently each befriends the turtle keeper, George Fairburn, and asks about freeing the turtles. Surprisingly Fairburn is amenable to the idea.
William and Neara meet when Neara goes to William's bookshop. Each recognizes in the other a kindred spirit and though neither is seeking to make a connection they reluctantly decide to collaborate to help Fairburn free the turtles.
On the appointed day, William and Neara rent a truck and travel to Polperro where Neara had suggested launching the turtles, unaware that the town is the one William was born in. Their mission is a success and they are both briefly elated, though returning to London both drop their connection.
Each feels that despite their initial euphoria the turtles have not changed them in any substantial way. However Neara begins a relationship with George while William ends one he had with his coworker, Harriet, and physically fights Sandor, one of the fellow boarders at his rooming house.