Jaimie Vane's promising career as a cellist came to a screeching halt when chronic illness took over her life, leaving her dealing with a disability her doctors don't understand. Then her vibrant sister Sybil, an influencer turned wellness consultant at an exclusive health retreat, leaves her a series of distressing messages that contain hidden cries for help...and a voucher for a free stay. Determined to find her sister, Jaimie finds herself chartering a tugboat across the frigid winter sea to the black-sand shores of Creag Dhubh, the tiny island that houses the elite Saoghal Spa and its enigmatic founder, Cyan Blache. Searching for her sister under the premise of being a regular guest, Jaimie is pushed far out of her comfort zone, both physically and mentally, by the group healing activities and luxurious treatments. As time wears on and Sybil is nowhere to be found, Jaimie is shocked to find herself feeling...better. There is something at Saoghal, in the treatments and tinctures and the black mud itself, that has her chronic fatigue and pain melting away. And as she gets closer to the people around her, including the ethereal Cyan Blache, whose perfection seems to defy reality, she finds herself with more questions than answers. But everything comes at a price, and transformation, it seems, requires sacrifice. When Jaimie's investigation leads her to discover that something darker than mud baths and salt scrubs is twisting the residents of Creag Dhubh into something more than themselves, she must face an evil beyond reckoning in order to escape with her life.