When her scientist mum disappears one night, Anya sets out to find her in the middle of a storm. Taking a boat out to sea, she encounters a strange window of light hovering above the waves. Transfixed, she reaches out to touch it. Moments later, Anya wakes in her boat. The sea now eerily silent. With her mum still missing, Anya is packed off to stay with the odd, unwelcoming Mr and Mrs Ungsworth. Trapped under their watchful gaze, she uncovers a shocking secret: the light has split her into two people. Anya escapes and returns home – to find another girl, identical to her, living there. Confronted with this impossible situation, the girls soon realise the organisation their mum worked for, Janus, is seeking to harness the power of the mysterious light and is now after them. Thrown headfirst into the perilous world of Janus, both versions of Anya must now team up to escape them. But can Anya’s battle to find herself also lead her back to her mother?