Isabel is on a plane heading back to her island birthplace in Papua New Guinea. Izzy is looking forward to seeing her family again, but there's another tragic reason for the trip. Izzy's twin brother, Ray, died in a freak diving accident, and Izzy and her mum are taking his ashes home for traditional death ceremonies.

After they arrive, Izzy realises things have changed since their last visit. Logging threatens the community's way of life and sharks no longer answer the song of the shark callers.

Izzy's cousin Noah explains that the clan needs someone to undertake a traditional diving ritual. The person must be a twin from the shark calling lineage. The dive will be perilous.

And Izzy is the last twin.

Will she have the courage to attempt the dive? And what deep, dark secrets will the ocean reveal if she does?