Ali Dawson and her cold case team investigate crimes so old, they're frozen - or so their inside joke goes. Most people don't know that they travel back in time to complete their research. The latest assignment sees Ali venture back farther than they have dared before: to 1850s London in order to clear the name of Cain Templeton, the eccentric great-grandfather of MP Isaac Templeton. Rumour has it that Cain was part of a sinister group called The Collectors; to become a member, you had to kill a woman. Fearing for her safety in the middle of a freezing Victorian winter, Ali finds herself stuck in time, unable to make her way back to her life, her beloved colleagues, and her son, Finn, who suddenly finds himself in legal trouble in the present day. Could the two cases be connected?