Ten-year-old Jasper has been waiting all spring for his beloved nightingale to return to his garden and sing. But it's not there, and neither is his sister, Rosie. His parents seem sad and preoccupied, so gathering his courage, his backpack and his treasured Book of Birds, Jasper sets out alone on a walk to find them both.

The expedition takes Jasper through town and country, meeting a host of characters who are also searching for lost things. Helping his new friends, Jasper begins to see that he may not find what he is looking for when he reaches the journey's end, but even in the darkest of moments, a nightingale's song can be heard somewhere.