Once upon a time, the isles of Wales were rife with magic and conflict - and eighteen-year-old Mererid "Mer" is well-acquainted with both. As the last living water diviner, she possesses the ability to manipulate water - a unique elemental power that many will kill to possess.

She has spent years running from the prince who bound her into his service when she was only a child and forced her to kill thousands with her magic. Now, all Mer truly wants is to find a way to live a safe, quiet life.

But when Mer's old handler-the king's spymaster-returns with a proposition: use her powers to bring down the very prince that has abused them both; Mer must decide if she's prepared to run for the rest of her life, or fight for her freedom and peace. So, with the help of a fae-cursed young man, a snarky thief, and a corgi that may or may not be a spy, Mer plans the perfect revenge.