Selina Penaluna is a merrymaid - or so she believes... Taken as a baby to the seashore, she slips from her mother's grasp and plunges deep into a rockpool. When her mother finally gets a hold of the slippery creature, she finds a mermaid child looking back at her - her baby taken and replaced by a mermaid changeling... Ellen and Jack are twins, evacuated from the East End of London to Cornwall during the Second World War. The family that takes them in are middle-class and a little stuffy but Ellen relishes the opportunity to better herself. But Jack is different. He finds their attentions stifling and seeks freedom in the arms of Selina, the mysterious local girl he sees at the shore. Selina, Ellen and Jack's lives are intertwined in a series of events that lead to tragedy. Could Selina have finally reverted to her merrymaid roots?