Brutus (James Mason) is convinced by a scheming band of Roman senators, led by Caius Cassius (John Gielgud), that his dear friend Julius Caesar (Louis Calhern) intends to dissolve the republic and install himself as monarch, and he joins a conspiracy to assassinate him. Brutus stirringly defends his actions, but when Mark Antony (Marlon Brando) responds with a speech that plays upon the crowd's love for their fallen leader, a battle between the two factions is assured.