Based on the play by George Bernard Shaw, Christians are captured to be killed in the arena for the entertainment of the Romans, but the results are contrary to expectation.
At Rome in 161 Cato (John Hoyt) tells the gladiator Editor (Alan Mowbray) that he is going to round up Christians in Syracuse for the circus. Shrewish Megaera (Elsa Lanchester) warns her husband Androcles (Alan Young) that he may be captured. He carries a large bundle, and they stop to rest. They see a lion, and she faints. Androcles removes a thorn from its paw. Megaera flees as soldiers arrest Androcles, the sorcerer.
The Captain (Victor Mature) sees beautiful Lavinia (Jean Simmons) with the captured Christians. Androcles arrives and meets Lavinia. The Captain sees his men being friendly with the Christians and reprimands them. Chained Ferrovius (Robert Newton) lifts a wagon to free a crushed man. The Captain orders his chains removed because he is a Christian. The Captain talks with Lavinia.
Courtier Lentulus (Reginald Gardiner) slaps Ferrovius, who only talks to Lentulus. Caesar (Maurice Evans) tells the Editor that the martyrs cause more people to become Christians. They arrive at the Coliseum.
Cato is cynical about the religious Spintho (Noel Willman), and Caesar sends Spintho home. A centurion (Jim Backus) catches Spintho stealing from the temple and puts him with the Christians. Spintho wants to be a martyr so that he will go to heaven. Androcles believes that animals go to heaven too. Lavinia is taken to the Captain, who asks her to accept Roman religion. He says that her courage is pride. She thanks him for trying to save her, and he kisses her.
Gladiators fight in the arena. The Editor asks Androcles to save himself. Androcles declines, but Spintho wants to sacrifice to the gods to save himself. Ferrovius gets angry at the Editor, and the Menagerie Keeper (Gene Lockhart) says that a lion ate Spintho. Androcles tells Caesar that he can live with his wife without beating her. Caesar offers Ferrovius a place in his praetorian guard if he wins. Androcles refuses to fight. Ferrovius takes a sword and declines armor.
The men go into the arena, and Ferrovius defeats several gladiators. The Captain pleads with Lavinia to burn incense and asks her to marry him. Lavinia says she will die for God. Caesar says that Ferrovius killed six gladiators and frees the Christians. Androcles is sent to the lion as a sorcerer. He kneels and prays, and the lion roars. They recognize each other and dance together.
Inside Androcles tells Caesar not to be afraid of animals and rescues him from the lion. Caesar tames the lion and shows others. Ferrovius agrees to be in the guard. The Keeper asks Caesar to make Androcles a slave in the Menagerie, but Androcles dislikes cages and is allowed to leave with the lion.
This spiritual parable of how love and gentleness can conquer anger and violence reflects also the vegetarianism of Shaw in his plea for humane treatment of animals.