Brutus contemplates the conspiracy in his garden late into the night. He has reached the conclusion that Julius Caesar must die. Brutus can't justify Caesar's death by any personal acts of Caesar's; Caesar has just got to go for the public good.
Brutus reasons that, although Caesar isn't bad now, getting a crown would change his nature. Brutus admits he's seen no evidence that ambition would change Caesar, but he reckons it isn't worth taking the chance.
Thus Brutus decides action must be taken now, as Caesar is like a serpent's egg – dangerous once hatched. (Time to make an omelet.)
While doing all this thinking, Brutus sends his servant Lucius to light a candle in his room. Lucius returns with a letter he's found (Cassius's invention). The letter says Brutus should recognize his own noble nature and do something before Rome falls to the tyranny of a monarch. Brutus is taken in and promises that, for Rome's sake, he won't fail.
Lucius then confirms that it's the Ides of March (the fateful day Caesar had been warned about). After this healthy bit of foreshadowing for the audience, Brutus admits he's been kept up every night since Cassius planted the fear of tyranny in his mind.
The group of conspirators then shows up at Brutus's door to try to win Brutus over to their cause. They're all disguised and looking shady.
Cassius introduces all the conspirators, and Brutus asks to hold everyone's hand for the Roman version of Kumbaya over their murdering plan.
Cassius suggests they swear an oath to their cause, which Brutus opposes violently. They are Romans, and Romans don't do oaths – they're just true to their word, even if that word is murder.
Then they all have a little debate about whether to include Cicero, but it's decided he'd never be a follower and shouldn't be invited to join Team Secret Conspiracy.
It's important here to note that the minor conspirators are easily swayed one direction or another regarding whether Cicero should be asked to join, at first thinking he'd be great and then insisting he's totally unfit. They're easily persuaded.
Cassius then suggests they also kill Antony (Caesar's young friend) while they're at it, though Brutus thinks this would be overkill. (Get it? Ugh.) He talks about how they should murder Caesar nobly, carving him up like a dish for the gods, not like a "carcass fit for hounds." The conspirators should think of the murder as an act of sacrifice for the state and not as a bloodbath.
Brutus also contends that because Antony is like Caesar's arm, once they kill Caesar, Antony will be powerless. An arm without a head can do nothing, and Brutus is sure they have nothing to fear from Caesar's friend.
Trebonius, another conspiratorial lackey, suggests that Antony will be sad after the murder but will eventually laugh about the whole thing. (Right.)
The clock strikes 3 (actually, ancient Rome had no clocks, but Shakespeare wasn't much of a historian), and they agree to part. Before they do, Cassius points out that Caesar has been cautious lately because of all the bad omens floating about. Cassius further worries that Caesar's prophets might convince him to take a sick day from the Capitol.
Decius tells everyone not to worry; he'll show up at Caesar's place in the morning to make sure Caesar goes the Capitol. He can sway Caesar easily with fairy-tale interpretations of whatever worries Caesar.
In fact, everyone will meet at Caesar's to make sure he shows up at the Capitol for the murder. It's a team effort. Cassius prompts them to be "good Romans" and keep their word.
Brutus tells them to make sure they don't look like suspicious murderers. Brilliant!
After everyone leaves Brutus, his wife, Portia, whom he left in bed, shows up to have a little husband-wife chat. The other night Brutus gave her a mean look at dinner and dismissed her when she wanted to talk about what was bothering him. (Apparently the plan to murder Caesar didn't make it into pillow talk.) Portia pleads with him to tell her what's making him so unhappy.
Brutus claims he's just a bit sick, and Portia says that pacing about at all hours of the night is surely not the best cure. She points out it must be a sickness of the mind that plagues him. Brilliant!
She says she has a right to know who the masked men who were just at their house in the middle of the night were.
Portia claims she does more than simply serve Brutus, and she asks that he confide in her as a beloved wife rather than ignore her like a kept woman. Though she knows she's a woman, she's his wife and the daughter of noble Cato, and she can keep a secret, no matter what it is.
Brutus then asks the gods to make him worthy of such a noble wife. Just then, there's a knock at the door. Brutus sends Portia back to bed, promising to tell her everything later.
Caius Ligarius, a guy who one of the conspirators wanted to bring onto the team, has shown up. Although he's sick, he says he's filled with spirit after hearing of the killing plan. The two walk and talk about the murder afoot.