At home in England, Darling is the mischievous but much-loved pet of Robert and Katherine. But when the British military asks families to volunteer their dogs to help the war effort, they send Darling off to be trained, even though it is very hard to say goodbye. Darling goes through training along with many other dogs and is ultimately used as a mercy dog, seeking out injured soldiers on the battlefield and leading the medics to them. After saving the lives of numerous soldiers, Darling is faced with a major challenge.