Brutus, unable to sleep, paces in his courtyard. He orders his servant Lucius to light a candle in his study. Alone, Brutus admits that the only possible course of action is to kill Caesar. He adds that, while Caesar's behavior so far gives no excuse for murder, it seems likely that absolute power will change him.
While the other conspirators fear reprisal and punishment, Brutus only fears whether killing Caesar is in the best interests of Rome. Ironically, Brutus is the first of them to explicitly state that Caesar must be killed.
Lucius returns and hands Brutus a letter he found. Brutus asks him to go check whether the next day is the ides of March, and reads the letter by the light of a meteor shower. It asks "Shall Rome, et cetera?" and urges him to "Speak, strike, redress" (2.1.46-7). Brutus takes this to mean that Rome must not have a king, and that he, like his ancestor, must prevent this.
Meteors were supposed, even in Shakespeare's time, to herald important events. As he does in conversation, Cassius leaves blanks in his letter. Brutus fills in the gaps—without his interpretation, the letter is meaningless.
Lucius returns to say that the next day is indeed the ides of March. There is a knock and Brutus sends Lucius to the door. Alone, he says that he hasn't slept since Cassius brought up the idea of moving against Caesar, and that the time leading up to a horrible deed feels like a rebellion within the body.
Brutus here symbolizes all of Rome—sleep represents peace, physical abilities represent governmental powers, and indecision, here brought on by a crisis of conscience, represents rebellion.
Cassius is admitted, with Casca, Decius, Cinna, Metellus, and Trebonius. Cassius whispers with Brutus, and then suggests they all swear an oath to follow through with their plans. Brutus says an oath should not be necessary, since the well-being of Rome, and the fact that they've already given their words, should be enough to motivate any true Roman.
Once Brutus decides that killing Caesar is necessary, he is unwavering. Because he's motivated by his morality, he rejects any suggestion that makes the conspiracy seem underhanded, such as the need to be bound by an oath.
Cassius suggests they ask Cicero to join them, and Metellus says that Cicero's venerability and known wisdom will make them look better. Brutus says that Cicero is too proud to take part in any plan that was someone else's idea.
The second suggestion of Cassius's that Brutus overrules. He, rather than Cassius, is beginning to seem like the leader of the conspirators.
After Decius asks whether only Caesar will be killed, Cassius suggests they kill Antony as well, since he may oppose them afterwards. Brutus says that without Caesar, Antony will be harmless, and more likely to kill himself out of grief than anything else.
Murdering Antony would be motivated by concern for their safety, not the good of Rome, and therefore is not morally defensible. This decision will have dire consequences.
Cassius says that Caesar's superstitions may keep him away from the Capitol, and Decius offers to go to Caesar in the morning, as he knows how to persuade him. Metellus suggests that Ligarius should be brought into their confidence, and Brutus says that he'll take care of this. The conspirators part.
More examples of an uncertain future: the omens might convince Caesar not to go; Decius might not be able to persuade him. Brutus is now in charge and takes responsibility for recruiting Ligarius.
Brutus's wife Portia enters, and questions him about the visitors and his strange behavior. He makes excuses, but she sees through them. Portia kneels before Brutus, but he asks her to rise. She argues that if he won't bring her into his confidence, then she is not truly his wife. She argues that she is stronger than other women, based on the noble characters of her father and husband, and reveals that she's stabbed herself in the thigh to prove her fortitude.
Like the male characters, Portia makes skillful arguments based on accepted ideas, e.g. the rights of a wife. Yet she uses her relationships to men, not her own merit, as evidence of her strength. The wound in the thigh combines violence with sexual overtones, symbolizing Portia's wish to be trusted like a man.
There is a knock. Brutus promises to reveal his secrets to Portia, who goes inside. Ligarius enters, and seems to suspect what is being planned. Brutus leads him inside, and Ligarius says that Brutus's involvement in the plot is sufficient evidence that it's a good idea.
Though Brutus decided that the plot was legitimate based on logic, Ligarius makes his decision based on what Brutus does. Through his virtue, Brutus unintentionally makes his poor decisions seem virtuous.