In a Welsh coal-mining town in the late 19th century, the Morgan family is headed by father Donald Crisp, Best Actor) and his wife Sara Allgood (nominated for Best Actress). There's also daughter Maureen O'Hara, four sons who are also coal-miners, and young schoolboy Roddy McDowell.

There is much good about this film. The script is so good that the dialogue sounds like poetry. The plot keeps things moving. Crisp, Allgood, and McDowell are given broad characters and perform flawlessly.

It's not quite an outstanding movie, however. One problem is that the four coal-mining sons are all rugged, manly, and interchangeable. O'Hara is in love with the minister Walter Pidgeon, and their forbidden and platonic romance, complete with long looks and stilted dialogue, damages the film. Also, the miners don't act like you would expect. I can believe that they walk down the streets singing in unison, but they never curse, boast, tell lewd jokes, talk with a harsh accent, lose their temper or otherwise act human.

One interesting casting is the hateful, shrewish minister Cyfartha, played by Barry Fitzgerald. Fitzgerald typically plays cantankerous but lovable Irishmen, so he is being cast against type here.

A surplus of workers causes the mine owners to reduce wages. The miners form a union and strike. This pits Crisp, who opposes unions, against his pro-union sons. As the strike wears on, bitterness by the town's coal-miners against Crisp leads to Allgood and McDowell falling to a frozen lake. They are saved and survive, but McDowell is left unable to walk.

Pidgeon visits the Morgan household to cheer up McDowell and tease O'Hara. With his help, he learns to walk again. Later, the wealthy mine owner's son visits the Morgan family to propose to O'Hara. The family must force her into marriage, since her heart belongs to Pidgeon.

Now fully recovered, McDowell is admitted to an elite school out of town. He is the only coal-mining son, and so is bullied not only by classmates but by the sadistic headmaster. Upon walking home after being bloodied in a fight, McDowell is taught how to box by his family. Now McDowell can outbox the school bullies, winning their respect, but the cruel headmaster has him caned.

Again walking home bloodied, a few of the coal-miners visit the school to confront the headmaster, beating him unconscious in front of his class. Surprisingly, this scene is almost a comedy, and it works.

After graduating, his father wants him to become a doctor or lawyer, but McDowell wants to prove his manhood and demonstrate his respect for the family by becoming a coal-miner. Mother is pleased with this but Crisp is disappointed.

The coal mining sons begin to leave home, as they are downsized by the mine owners. The married son dies in a mining accident, and the others leave the country for better opportunity. O'Hara's marriage fails, and there is gossip that it is because she has liasons with Pidgeon. Pidgeon confronts his congregation, and lambasts them for their ignorance.

There is another accident at the coal mine. Bad luck would have it that it is another Morgan, this time Crisp. Pidgeon and McDowell must go down into the mine to retrieve Crisp, who dies in McDowell's arms.

The director decides not to leave the movie on such a downer, so he adds a gratuitous, tear-jerking scene where the entire Morgan family is seen reunited on a valley.