Following their father's death, Cass and her sister Sid will do whatever it takes to keep their family's dive school afloat. It used to be the go-to destination in Cornwall for those keen to explore the ocean's hidden depths. But when Cass returns from a long trip, she's shocked to discover how bad things have become. The business is in decline. Sid's new boyfriend has far too much control over the accounts - and her sister. Then, Cass discovers a body in the water. And as more girls go missing, she wonders just how far someone will go to keep their secrets buried.