The story is set in 1970s Normandy, France. It stars Jean Gabin as Auguste Maroilleur, an aging landowner and farmer who rules his family and farm with an iron hand. One day, he finds out his grandson Henri was working as a courier for a drug-trafficking gang, hiding heroin on the family's lands. Auguste destroys the drug, whips his grandson as punishment and kills one of the gang's members who dared threatening him and his family. The gang then starts attacking the farm. Auguste decides not to call the police, choosing to face them instead.
"Horse" was a French slang word for "heroin", hence the title.
- "Ass" in Ambassador: Marc (the first gangster that comes to the farm) casually threatens to rape Auguste's granddaughter if the Maroilleur family doesn't surrender Henri and the drug Henri was hiding. Auguste shoots him in the chest right after he said this.
- Badass Grandpa: Auguste is an old farmer, but don't think he's a pushover. He's not afraid of the gangsters one bit, and he will prove them why.
- Drugs Are Bad: Upon discovering that his grandson Henri is involved in drug trafficking, Auguste whips him with a small branch out of anger, berates him for abandoning his literature studies and throws the heroin in a barrel of water.
- Farmer's Daughter: Auguste's granddaughter is a French countryside version. She fits the "kept at home under very strict rules" part.
- Lured into a Trap: Auguste and his sons-in-law agree to meet the gangsters. Auguste comes alone to meet them at dawn while his sons-in-law hide in water tanks to ambush the bastards, leaving them no chance of escaping.
- Meta Casting: Jean Gabin was a landowner just like Auguste.
- Molotov Cocktail: How the last gangster is disposed of. August shoots in his face as the car passes by, and one of the sons-in-law throws a Molotov cocktail at the car.
- The Patriarch: Auguste is an old farmer, but he's also a strong patriarch who will go to great lengths to defend his family, as well as their honor and lands. No one ever dares to contradict him or raise a voice against him, and he rules the family with an iron hand. The movie doesn't shows this as a bad thing, as his grandson screwed up badly by neglecting the farm and his studies and getting involved in the criminal underworld.
- Perpetual Frowner: Auguste never smiles.
- Rape Is a Special Kind of Evil: Marc's threat concerning Auguste's granddaughter if they don't surrender Henri and the drug. The gang indeed rapes her later on.
- Title Drop: Auguste and one of his sons discover heroin hidden in the bird observatory of their lands after the film's opening titles, and the son namely refers to it as "horse".
- Too Dumb to Live: Marc threatens to have Auguste's granddaughter raped, right in front of the old man who carries a hunting shotgun. He ends up with 12-gauge in his chest for it.
- Vigilante Man: Auguste and by extension all the adult men in his family, as they decide to get rid of the gang by themselves without calling the police, as it would draw attention on Henri (since he is part of the drug ring and as such would get arrested) and taint the family's honor.