It is now spring 1939 and a potential world war looms. Tristan is now a qualified vet, while James enters a new stage of his life with Helen and at the practice. After being made joint business partner by Siegfried, James pushes to take on more responsibility via the Ministry of Agriculture's new bovine TB testing scheme, but its unexpected challenges risk pushing him to breaking point.

Everyone is trying to get along as best they can in the face of changes at Skeldale House. Helen has moved into the bedsit upstairs with James. James struggles to get Siegfried to let him take the reins now that he is a junior partner, and the pair clash when he learns how much money Skeldale could make from the Ministry of Agriculture’s lucrative bovine TB testing scheme.

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 Agriculture’s TB testing scheme, but is struggling with the complicated paperwork - a fact that he is keen to keep from Siegfried. Tristan decides that he is outgrowing running the small animal surgery and buys a car to help with errands at the practice, which also provides him with transport for taking Florence, the daughter of Siegfried’s rival, George Pandhi, out on a date.

Siegfried and Tristan are left in charge of Skeldale House, 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 and focuses on running the practice and educating a visiting work experience boy named Andrew Simmonds.

It’s Christmas in Darrowby and everyone is trying to make the most of things while the world is at war. A young guest brings some wonder, mischief and cheer to Skeldale House while everyone waits for important news. When Mrs Pumphrey heralds the arrival of a kitten in need of extra care. Siegfried is asked to pay a visit to an injured River just before a big race.