When the play opens, Julius Caesar has just returned to Rome after defeating the sons of Pompey in battle.
Before we go any further, let's pause for a brief Roman history lesson. Pompey (a.k.a. "Pompey the Great") was a member of the "first triumvirate," and he and Caesar used to share power over Rome. Then Caesar and Pompey got into a big fight. Pompey lost. When he tried to run away to Egypt in 48 B.C., he was assassinated. But Caesar still had a problem: Pompey's sons were determined to avenge their father's death and overthrow Caesar. So Caesar tracked down Pompey's sons in Spain and stomped them out at the Battle of Munda in 45 B.C. Now back to the play.
As Caesar parades through the streets of Rome like a rock star, the higher-ups in Rome are nervous about his growing power and his popularity with the commoners, who have abandoned their work to celebrate Caesar's triumphant return. Caesar seems headed toward absolute power, which is a big no-no in the Roman Republic.
Meanwhile, the festival of the Lupercal (a big party where people run around in goatskin g-strings in the middle of February) is in full swing. Caesar is chilling at the festival with his entourage when a soothsayer runs up and says "beware the Ides of March" (meaning, "hey, watch your back on March 15"). Caesar looks at the soothsayer and is all "whatever man."
While Caesar parties with his fans, Brutus and Cassius huddle together and talk trash about him. Cassius is all bent out of shape because he thinks Caesar is running around acting like a king. Without coming right out and saying so directly, Cassius (who has been plotting against Caesar with a group of conspirators) suggests that maybe Brutus should lead Rome. Brutus says he gets what Cassius is saying, but he is also good friends with Caesar, so he needs a little time to think about things before he makes any decisions. (Psst. If you read the play closely, there's some evidence that Brutus has already been thinking about getting rid of Caesar, because he confesses that he's been "at war" with himself, meaning something's been bothering him.)
Brutus and Cassius run into Casca, a conspirator, who reports that Antony just offered Caesar the crown three times. Casca is mad, because each time Caesar pretended he didn't want the crown, which made the crowd of plebeians (common folk) love him even more. Not only that, but Caesar acted like a total drama queen and fainted (or pretended to) the third time Antony offered him the crown. This made the "stinking" crowd go nuts.
A month passes, which means we're approaching the "Ides of March." (Cue the ominous music.) Casca and Cicero are running around in a violent thunderstorm and comment on all the crazy stuff that's been happening in Rome lately: a lion was roaming around and a bunch of men in flames were spotted walking around the streets. Cassius, who interprets these omens to mean that Caesar must be taken down, continues to plot against Caesar. He sends someone to plant fake letters from Roman commoners urging Brutus to eliminate Caesar, and attends a meeting that night to plot Caesar's death.
Meanwhile, Brutus has decided to go ahead and kill his friend Caesar because the man might become a complete tyrant if he gains more power. Brutus reasons that, even though he and Caesar are BFFs, killing Caesar is the only way to save the Roman Republic. (Is he right? We don't know for sure, but Shakespeare definitely wants us to think about this.) Brutus finally meets with all the conspirators, and they hatch a plan: they'll arrange to bring Caesar to the Capitol so they can hack him into a million little pieces.
Meanwhile, Caesar has had a rough night, complete with a crying wife (Calphurnia) who wants Caesar to stay at home because she's had a bad dream and fears something awful is about to happen to him. But Caesar ultimately decides to go to the Capitol, because Decius (one of the conspirators!) steps in and says something like, "Oh, hey, when Calphurnia dreamed that you were a statue full of holes and spouting blood, that just meant that you're going to be the greatest leader Rome has ever seen." Caesar is all, "Yeah, I think you're right." Decius promises that Caesar's going to be crowned king that day. Caesar goes skipping off to the Senate. On the way to the Capitol, an old man tries to give Caesar a letter warning him about the assassination plot, but Caesar blows him off.
At the Capitol, Caesar stands around bragging about how awesome he is. Just as he's making a big speech about how he's the brightest star in the sky, Cassius, Brutus, and the other plotters surround him and stab him to death – 33 times, just to be sure. Before falling, Caesar looks up and says "Et tu, Brute?" Translation: "Even you, Brutus? What happened to us being best buds forever?"
The conspirators wash their hands in Caesar's blood (hmm...seems like Calphurnia's dream was pretty accurate after all) so they can walk the streets and calmly tell everyone that Rome is free of tyranny. The idea is that they'll seem more convincing about their plans for a new dawn of peace if they're dripping with Caesar's fresh blood. Surprisingly, instead of hailing Brutus and Cassius as saviors, the people of Rome run around declaring that it's Doomsday. The situation is not going according to plan.
Things really go awry when Antony shows up to weep over Caesar's body. While clearly distraught, he promises not to blame the conspirators as long as he's allowed to speak at the funeral in praise of Caesar's virtues. Of course, we hear in an aside that Antony plans mayhem and murder, so we're not surprised when he gets to the funeral pulpit and urges the people of Rome to riot against Julius Caesar's murderers. (An "aside," by the way, is when a character says something to the audience that no other characters on stage can hear.)
Meanwhile, Brutus and Cassius have fled and chaos has ensued. Even politically unimportant folks like poets are being killed on the street. Antony has met up with Lepidus and Caesar's adopted son, Octavius. Together they'll form the new triumvirate to lead Rome and battle against Cassius and Brutus.
Meanwhile, Cassius and Brutus get into a big argument at their first meeting after the funeral. Cassius has been accepting bribes on the side, which compromises their credibility. (Remember, the only reason Brutus agreed to join the conspiracy was that he believed killing Caesar was for the greater good, not for any self-serving reason. At least, that's what Brutus says.) Still, they agree to march and meet the enemy (Antony, Octavius, and Lepidus) at Philippi, despite a visit from Caesar's ghost to Brutus to say he'll be at Philippi too. It's going to be like a family reunion, except this one will mostly end in death. Everyone has steeled himself for this possibility, and Cassius and Brutus implicitly agree to pull a Romeo and Juliet (kill themselves) in case anything goes wrong in the battle.
On the battlefield the two enemy factions exchange some rough words, and Brutus claims he's not a traitor. Fighting ensues, and Cassius and Brutus set up on different parts of the field. Brutus is having some success in overtaking Octavius' army, but Cassius' guys are held fast by Antony's, so they're at a stalemate.
Then Cassius jumps the gun and kills himself over a misunderstanding: he thought his friend Titinius had been overtaken by enemy hordes, when it was really only Brutus' friends trying to hand a crown to Titinius so he could give it to Cassius. Titinius finds Cassius' body and kills himself too, so when Brutus arrives, his buddies are already dead. Then Brutus decides to kill himself. He gets his old friend Strato to hold his sword while he runs at it. As he dies, he says he didn't kill Caesar with half so strong a will as he kills himself now, so we know he dies willingly.
Antony and Octavius know they've won even before they arrive to find Brutus' body. Antony gives a nice speech over the body in his usual style, saying Brutus was the noblest Roman ever and the only one of the conspirators who killed Caesar for Rome's good and not out of envy. Finally, Octavius agrees that Brutus's body can stay in his tent for the night, befitting a dead soldier, and they won't even have to share a bunk, as Octavius and his friends will be out celebrating all the death and victory. The end.