Set on the Welsh coast. Stella Bevan, aged eleven, is helping her elder brothers build a boat. They are going to call it The Evening Star. Stella is expecting a French girl to stay with her during the holidays, but Philippe, a brother of the French girl, arrives instead. He proves (to everyone's surprise) exceptionally good value, not only in helping to launch the boat but also in sailing it. All the Bevan family subsequently become involved in a local injustice which they put right, and in which Philippe plays an audacious and decisive part as a detective.