In Inveranoch, Scotland, in the year 1912, veterinarian Andrew MacDhui lives with his seven-year-old daughter Mary and her cat Thomasina, who narrates the film in voiceover.

When Thomasina contracts tetanus after a scuffle near the fish market goes wrong, MacDhui orders his assistant Willie Bannock to euthanize her.

Willie reminds MacDhui that he promised Mary to make Thomasina well again, but he is so preoccupied with his surgery on another patient's seeing eye dog that he ignores the plea.

Mary and her friends take Thomasina's body beyond the town for a funeral, but they are frightened by the appearance of "Mad Lori" MacGregor, a young woman who lives in the glen and was attracted by the children's singing and bagpipe playing.

MacDhui visits Lori with the intention of confronting her for stealing his business, but instead they both realize that they each have half of what is needed to treat sick animals: he has the science and surgical knowledge; she has the love and compassion.

When MacDhui and Lori discover the Romani have been abusing their performing animals (for instance, a dancing Asiatic black bear), they visit the circus.

He thought it was a "sentimental and extremely genteel little movie... best suited for small girls," but praised the major performers (including the cat) and the settings.

Maltin also said Dotrice "won over everyone" with her performance in Thomasina, and she (and fellow cast member Matthew Garber) were signed to play the Banks children in the Disney film Mary Poppins.