William resents going on a 'boring' holiday to Wales, but while there he is let into a big secret by Gwyn, a local boy. Gwyn has found a seal pup and has it hidden in a cave. He makes William promise never to tell anyone about it, and plans to breed it on a farm. William is determined to let it go free, and 'borrows' a leaky dinghy to take it out to sea. The story ends with an exciting rescue.