Martin can't believe his eyes when a strange girl staggers out of the sea. 'How can I go home now my sea-cloak is in tatters?' she cries. 'When I wore it I could fly, walk and swim to my home at the bottom of the sea.' Martin and his mother decide to help Una, and Martin searches far and wide for the things they need to repair Una's cloak: silver grasses...tiny green feathers...rare shells...Then comes the day when Una can at last fly away, and soon Martin begins to miss his new friend.