A doctor in the hills of Scotland buys a dog and helps a young man learn medicine while treating people in the area.
Dr. William MacLure (Edmund Gwenn) delivers a baby and treats a sick man. Mr. Milton (Rhys Williams) sells Lassie to MacLure for all his unpaid bills. Tammas Milton (Tom Drake) wants to be a doctor, but his father wants him to farm. Hopps (Reginald Owen) tells MacLure that Lassie is afraid of water and is not a sheepdog. MacLure tries to get Lassie used to water, and they both fall in the stream. At his tavern Hopps makes fun of MacLure until Drumsheugh (Donald Crisp) gets the others to leave.
At the fair Dr. Weston (Frederick Worlock) tells MacLure about chloroform, and on settling day people pay MacLure what they owe him. In the snow MacLure and Lassie find a bridge is broken but get across without the horse. MacLure tells Belle Saunders (Eileen Erskine) that her husband may live. MacLure sends Lassie for Tammas, who helps with the cold bath and whiskey treatment.
MacLure tells Drumsheugh that he is training Tammas to succeed him. MacLure tells David Mitchell that his wife is dying. Margit Mitchell (Janet Leigh) gets upset that her mother is going to die. MacLure tells Drumsheugh he needs a hundred pounds for a doctor in London, and they agree to split the cost.
MacLure meets Dr. George (Alan Napier), and Lassie refuses to lead the carriage across a river. MacLure tells David that his wife will live, and Dr. George declines the fee.
Tammas has appendicitis. Dr. MacLure advises the use of chloroform, but Mr. Milton is against it. So MacLure shows Milton how to apply it to Lassie, and she comes back to consciousness as Milton is leaving. Mr. Milton applies the chloroform to Tammas, and MacLure operates. For his fee MacLure asks that Tammas be given four years at Edinburgh. Mr. Milton reluctantly agrees.
Years go by, and people help Dr. MacLure. At night MacLure goes out in the snow to treat a patient. Going home he falls off his horse. Lassie runs for help to Mr. Milton. Lassie astounds them by swimming across the river, and MacLure is rescued.
Drumsheugh visits MacLure, who says he is dying and instructs him what to do after he is gone. Tammas must not be told until he finishes school. MacLure asks Drumsheugh to pray, says goodnight, and dies. Drumsheugh puts out the fire, stops the clock, and reads from the Gospel. Lord Kilspindie comes to the funeral and offers to provide a tombstone. Tammas comes home to Margit, and they read the tombstone about the blessing of visiting the sick.
This family drama portrays a loyal dog and the rural
Scots and how they like to drink whiskey and argue about money
while showing how essential a doctor is in the country.