DI Shona Oliver watches in horror as her fellow officer is pronounced dead.

PC Hayley Campbell drowns during a triathlon race in the Solway Firth. The postmortem reveals drugs in Hayley's system, perhaps self-administered performance enhancers. But a puncture wound in the back of her wetsuit suggests something darker.

Shona makes slow progress on the case. There are other demands on her attention. The death of her old boss brings Shona under scrutiny. And the drug-related death of a teen at a festival – one which Becca is attending – has Shona on-edge.

Multiple people had a reason to want Hayley dead. Can Shona keep her head above water long enough to work out which one?