In late 19th-century Edinburgh, a baby named Jack is born with a frozen heart on the coldest day ever recorded. To save his life, midwife Madeleine replaces his heart with a cuckoo-clock mechanism, warning him to avoid strong emotions, especially love, to prevent the clock from breaking. Raised by Madeleine in a secluded environment, Jack’s curiosity and longing for human connection grow. On his tenth birthday, he ventures into the town for the first time and meets Miss Acacia, a spirited street singer. The encounter leaves him smitten, despite Madeleine's stern warnings. Obsessed with seeing her again, Jack enrolls in school, where he faces both friendship and intense rivalry, particularly from Joe, who bullies him and shares an interest in Miss Acacia.

Jack's quest to find Miss Acacia leads him on an extraordinary adventure across Europe. Along the way, he encounters a colorful cast of characters who challenge and shape him. Determined to win her heart despite his fragile condition, Jack ultimately embarks on a journey of self-discovery, struggling to balance his feelings with the need to protect his delicate heart. He learns about love, resilience, and the complexities of human emotions as he navigates through the testing trials that define his path forward.