DS Kat Ballantyne is still reeling from a revelation that turned life as she knew it upside down. But when a dog walker discovers the body of a woman among the standing stones known locally as the Three Sisters, there's no time to focus on her own troubles. Kat is drawn back to work to investigate.

At first, the death appears to be an accident. But then Kat discovers an infinity symbol branded onto the dead woman's abdomen. She's convinced there's a vicious killer at large. Still juggling her turbulent private life with her professional duty, she begins to dig into the victim's past. There, she uncovers shocking secrets and several potential suspects.

When a second woman is found killed in similar circumstances, Kat needs answers fast. Does she have a serial killer on her hands? Or is the perpetrator motivated by something more personal? What connects the victims? And can she catch the killer before anyone else turns up dead?