The rising tide / Ann Cleeves.

Fifty years ago, a group of teenagers spent a weekend on Holy Island, forging a bond that has lasted a lifetime. Now, they still return every five years to celebrate their friendship, and remember the friend they lost to the rising waters of the causeway at the first reunion. Now, when one of them is found hanged, Vera is called inches Learning that the dead man had recently been fired after misconduct allegations, Vera knows she must discover what the friends are hiding, and whether the events of many years before could have led to murder then, and now . . . But with the tide rising, secrets long-hidden are finding their way to the surface, and Vera and the team may find themselves in more danger than they could have believed possible . . .