It's spring 1939, as the period drama returns for a 3rd series. Tristan is now a qualified vet, and James enters a new stage of his life with Helen; however, with war looming, all have to consider their purpose in Darrowby and beyond.

James struggles to get Siegfried to let him share responsibility now that he's junior partner at the practice. With Helen's help, James learns how lucrative the Ministry of Agriculture's Bovine TB testing programme could be for Skeldale.

Siegfried receives some difficult news and becomes even more irascible than usual. The perfect distraction comes along in the form of Major Sebright Saunders, who asks Siegfried to look at a difficult young racehorse called River.

James is finally succeeding with the farmers' uptake of the Ministry of Ag's TB testing scheme. Tristan is outgrowing running the small animal surgery and buys a car to help with errands at the practice, enabling him also taking a date out

When Mrs Hall takes time off to meet up with an important person from her past, Siegfried immediately puts Tristan in charge of housekeeping duties, while he focuses on running the practice and educating a visiting work experience boy.

James finds a case of TB while testing his father-in-law's herd. Could it be the end of the farm? Meanwhile, Tristan proposes marriage while Mrs Hall tries to save an abandoned dog. Suddenly, Britain declares a state of war.

As Christmastime is overshadowed by war, the Farnons have taken in a refugee child. Siegfried must choose between the well being of a racehorse and safeguarding Tristan's future. Mrs. Hall also grapples with upsetting news.