They were best friends.

Now they are murderer and witness.

Pearl Day has always lived in the background - companion to her childhood friend, the dazzling and unpredictable Lady Eleanor Nicholson. Their bond was forged at Alderleigh, Eleanor's crumbling country estate, but now they share a sleek London home where Eleanor's life of indulgence is spiralling into chaos.

When Eleanor shoots her lover in a drunken rage, Pearl becomes the key witness in a scandalous murder trial. But she knows more than she's revealed - and with Eleanor behind bars, she sees a chance to escape her quiet desperation. Their bond, once forged in friendship, is now warped by grief, envy and power. And Eleanor's reach is long.

Set between 1930s London and the windswept Cornwall coast, this taut, gothic thriller dares to answer one of literature's abiding questions: Who is the woman in the water?