Edinburgh, late nineteenth century. Little Jack is born on a day so cold that his heart remains frozen. Madeleine the midwife replaces his heart with a cuckoo-clock. It will work, as long as Jack follows the rules, mainly not falling in love. But his encounter with a fiery-eyed girl singing on a street corner and his decision to chase after her will test the resistance of his makeshift heart to the breaking point.

In recovery from a life-threatening illness, Kate's work as an elf at a Christmas-themed shop begins to suffer as she keeps the secret from her employer and does not discuss it with her first-generation immigrant parents or her distanced sister. As the holidays draw closer, she encounters the same kind-hearted man repeatedly, and finds herself eventually warming to him despite his unreliable nature, letting his optimistic worldview gradually change...