Caesar is in fact a minor character in the action of the play, the main protagonists being Brutus and Cassius. Brutus is Caesar's close friend and a man of honour, who allows himself to be cajoled into joining the assassins because of a growing suspicion -- implanted by Cassius -- that Caesar intends to turn Rome into a monarchy under his own rule. Cassius, though not a villain, is motivated largely by envy.
The early scenes deal mainly with Brutus's arguments with Cassius and his struggle with his own conscience. After Caesar's death, however, another character appears on the scene, in the form of Caesar's devotee, Mark Antony, who, by a rousing speech over the corpse -- the much-quoted Friends, Romans, countrymen, lend me your ears... -- deftly turns public opinion against the assassins.
In the later scenes, Brutus and Cassius begin to fall out, Brutus having realised the error of his ways. As they prepare for war with Mark Antony and Caesar's great-nephew, Octavian, Caesar's ghost appears with a warning of defeat. In the course of the battle, Brutus and Cassius die, but the play ends with a tribute to Brutus, who has remained true to his own beliefs throughout. There are also hints at friction between Mark Antony and Octavian, preparing the audience for another of Shakespeare's Roman plays, Antony and Cleopatra.
A Swiss traveller in London, Thomas Platter, recorded seeing a performance of a play about Julius Caesar on September 21, 1599. He also reports the actors dancing a jig at the end of the play, a strange convention of the Elizabethan theatre.