Act I: Caesar has just emerged victorious in a series of Roman civil wars. The populous swarms to see his homecoming, but tribunes question the celebration. A Soothsayer foreshadows the play by giving Caesar a warning, which he ignores. Cassius beg ins to subtly sway Brutus against Caesar. The conspirators meet and decide they need Brutus to join them, for tomorrow they must kill Caesar before he becomes king.
Act II: Brutus joins, but Cicero is left out. Brutus foolishly decides they should not kill Mark Antony. Calpurnia tells her husband Caesar to stay home that day, but Caesar still goes to the senate.
Act III: The conspirators pretend to petition for a recall so that they may crowd around him, and then stab him to death. Caesar fights back at first, but when Brutus takes his turn, Caesar gives in dramatically. As the conspirators try to calm the city, Mark Antony steps in and wins Brutus over with flattery. Cassius fears him, but Brutus foolishly lets him speak to the crowds. At the funeral, Brutus gives a short but well-put speech and then his mistake proves costly. Antony riles up the crowd ag ainst the conspirators with a magnificent oration. Antony agrees to join Octavius Caesar and General Lepidus in a three-man government.
Act IV: Civil war now erupts between the new government and the conspirators. In Asia Minor, Cassius' army comes to join Brutus' army. Cassius and Brutus argue and make up. Brutus finds out that Portia is dead, along with many senators including Ci cero. Caesar's ghost visits Brutus and says they will meet again.
Act V: The armies sit opposite each other near Philippi, waiting for battle. Antony tells Cassius things might be better had he been in charge instead of Brutus. Cassius and Brutus exchange good-byes, knowing they may never see each other again. Br utus poorly leads his men, and turns a sure victory into a possible defeat. Cassius mistakenly thinks he is prisoner when in fact the conspirators are winning, and commits suicide. Brutus continues to mislead, avoiding a sure victory, and eventually it co sts him. He commits suicide in the face of defeat. Antony's forces win.
The summary of this play was taken from http://www.ocf.berkeley.edu/~anthony/Shake2.html