In 44 BC, Julius Caesar is bestriding the narrow world like a colossus. There are rumors that Caesar, who has already gotten himself made dictator-for-life, wants to drop the pretense and become king. Mark Antony, his lieutenant, offers him a crown three times at the Lupercal festival, and Caesar rejects the crown three times, but each time with more reluctance.
Gaius Cassius, one of Caesar's opponents during the civil wars, is not reconciled to Caesar's rule. Seemingly out of a mixture of envy, personal dislike, and principled opposition to dictatorship, Cassius recruits a group of Senators to murder Caesar. To add moral authority to his plot Cassius recruits Marcus Junius Brutus, a close friend of Caesar who nonetheless opposes Caesar's boundless ambition. Brutus finally agrees to join the plot.
On March 15 the conspirators get all stabby. They believe that with Caesar's murder they have destroyed the threat of tyranny, but they didn't count on Antony, who turns public opinion against them. Events climax with a great battle between pro- and anti-Caesarians at a place called Philippi.
The All-Star Cast features James Mason in the starring role of Brutus, Marlon Brando as Antony, John Gielgud as Cassius, Edmond O'Brien as Casca, Greer Garson as Calpurnia, Deborah Kerr as Portia, and Louis Calhern as Julius Caesar. Miklos Rozsa composed the score.
- Ambition Is Evil: Brutus and company kill Caesar because he's too ambitious. At least that's what Brutus says, and Brutus is an honorable man.
- Anachronism Stew: Besides all the anachronisms from the play (like chiming clocks), a title card describes the struggle for "the empire of the Romans", when what is now called the Roman Empire did not really start until Octavian/Augustus took sole control of the state.
- Antagonist in Mourning: The famous ending where Antony calls Brutus "the noblest Roman of them all" and says that while the other assassins acted out of jealousy or their own ambition, Brutus alone really acted on principle.
- AstroTurf: Cassius arranges for propaganda letters to be thrown through Brutus's window, with an eye to pushing him into the conspiracy.
- Blind Seer: The soothsayer who tells Caesar to "beware the Ides of March" is shown in the film to be a blind man.
- Blood on the Debate Floor: Caesar is stabbed to death in the Senate house.
- Call-Back: The lute that Lucius plays for Brutus is seen, broken in half, when Octavian's men enter the camp.
- Catapult Nightmare: Calpurnia bolts upright as she comes out of her nightmare, shouting "They murder Caesar!" This is a change from the play, where this scene is introduced with Caesar wandering around the house complaining about his wife's nightmare. (Probably the idea was to give Greer Garson a few more lines.)
- Could Say It, But...: How Antony lets the people of Rome know that Caesar left money to them, by saying "'Tis good you know not that you are his heirs."
- Death Glare: Cassius has a habit of doing these, most notably in the opening scene where he's shooting death rays with his eyes as Caesar talks with the soothsayer.
- Deliberately Monochrome: In the 1950s a lot of films were still being made in black and white but big Sword and Sandal films were getting made in color. MGM wanted this film to be in color, but Mankiewicz and producer John Houseman insisted on making it in black and white, aiming for the feel of a newsreel instead of a color Ancient Rome epic (like The Robe, made the same year).
- Died on Their Birthday: Cassius realizes that it's his birthday right before they ride into action at Philippi; he later takes his own life after the ensuing battle.
- The Dying Walk: Caesar staggers over toward Brutus before Brutus gives him one last stab in the gut, followed by "Et tu, Brute" and Caesar's death.
- Empathy Doll Shot: The lute that Lucius the servant boy played for Brutus is seen, broken in two, in the wreckage of Brutus's camp.
- Et Tu, Brute?: After his friend Brutus stabs him, Caesar gives up, says the Trope Namer line, and keels over in death.
- Good Is Dumb: He may be honorable but Brutus makes a lot of terrible decisions. He overrules Cassius three times: Brutus refuses to kill Antony, Brutus insists on letting Antony speak at the funeral, and Brutus insists on giving battle at Philippi immediately. All three decisions are calamitous errors.
- In Case You Forgot Who Wrote It: The title card says "William Shakespeare's Julius Caesar".
- Inertial Impalement: Brutus has his servant hold a sword, which Brutus impales himself on.
- Leaning on the Fourth Wall: As they're crouched around Caesar's corpse, Cassius says "How many ages hence shall this our lofty scene be acted over in states unborn and accents yet unknown?"
- Match Cut: A shot where the rioters are setting fires in the square after Antony's speech cuts to a far more peaceful fire, Caesar's funeral pyre.
- Meaningful Look: Antony turns away from the crowd two times during the "Friends, Romans, countrymen" speech. The first time he looks tense, as matters are obviously on a knife's edge. The second time he smirks, because he knows he's got them.
- Pragmatic Adaptation: A lot of Act V is cut, including the parley scene between Antony/Octavian and Brutus/Cassius, and Titinius discovering Cassius's body.
- The Purge: Antony, Octavian, and Lepidus make a list of people to kill.
- Rousing Speech: Marlon Brando delivers the famous "Friends, Romans, countrymen" speech, in which he whips up the crowd into a frenzy of rage against the assassins.
- Silence Is Golden: The whole battle of Philippi is portrayed in a five-minute scene without any dialogue. Brutus and Cassius's army is marching through a valley, totally unaware that Antony's army is taking position for an ambush in the hills on either side. A lot of cuts show all of Antony's spear carriers and archers getting ready, before they all let loose on Antony's signal. After this Antony's men come rushing down and there's some brawling, before the film cuts to the next scene where Cassius is wandering around after the battle's been lost.
- Slouch of Villainy: Antony casually slouches in a chair at dinner as he marks off a list of people to be murdered.