Brutus reflects in a soliloquy that he has nothing against Caesar personally, but Caesar must be killed for the general good of Rome. He explains that if Caesar is crowned king, that may change his nature, and he may abuse his power. He must be assassinated to prevent this possibility from taking place; in other words, Caesar must be killed not for what he is but for what he may become.
Brutus's servant Lucius brings him the letter that Casca has tossed in through the window. Brutus reads it out loud to himself and puzzles over the meaning. The letter urges him to take action for the sake of Rome, and it appears that he has made his decision to join the conspirators. Lucius enters and confirms that the next day is the ides of March. Brutus, alone, confesses that since Cassius first sounded him out about Caesar, he has not slept.
The conspirators, Cassius, Casca, Decius, Cinna, Metellus Cimber and Trebonius enter. Cassius demands that they take an oath to cement their resolve to assassinate Caesar. But Brutus refuses, saying that they need only their own cause to spur them onward. As Romans they are honor-bound to keep any promise they have made.
The conspirators then discuss whether to enlist Cicero in their cause. Cicero is old and wise, and his support would give their cause more legitimacy in the eyes of the people. But Brutus advises against this. He says Cicero will never join an enterprise that is led by anyone other than himself. The conspirators agree to leave him out of their plans. Next they discuss whether they should kill Antony as well as Caesar. Cassius advocates killing them both, because as a close friend of Caesar, Antony could prove troublesome to them. But Brutus says no; killing both men will make the conspirators seem too bloodthirsty, and anyway, Antony will be made powerless when Caesar is killed. Brutus hopes that if they go about their purpose with dignity rather than wrathful passion, the common people will see them not as murderers but as cleansers of the body politic.
Cassius remains uneasy, but the conspirators side with Brutus and decide not to kill Antony.
It is three in the morning and the conspirators agree to part. But Cassius is concerned that Caesar, whom he believes to be superstitious, may not go to the capital later that morning because of the strange apparitions during the night. Decius says he knows how to talk to Caesar and persuade him. He promises to bring Caesar to the capital. Cassius says they will all be there at eight in the morning to make sure that Caesar comes.
After the conspirators leave, Brutus's wife Portia enters. Brutus is surprised to see her up in the middle of the night. She knows Brutus is troubled by something and asks if she may know what it is. Brutus replies that he is sick, but Portia does not believe him. She knows that it is Brutus's mind that is troubled, and again demands to know what is wrong. She reassures him that she is strong enough to know the truth, and that she will not disclose it to anyone. Brutus is touched by her pleas, but then there is a knock at the door, and Brutus sends Portia away, saying that he will tell her everything later.
At the door is Caius Ligarius. Although he is sick, he is willing to join the conspirators.
The scene gives insight into Brutus's character. He thinks deeply about his decision and does not join the conspirators lightly. He thinks not of himself, or what he might gain from the assassination, but of the welfare of Rome. It is also clear that because he is the man most respected in Rome, the other conspirators defer to his judgments. However, Brutus is not always right, as is shown by his comments about how Antony should be spared because he will be powerless without Caesar.
The pleadings of Portia reveal Brutus's inner conflict. She is clearly a noble woman, a worthy wife for an honorable Roman, but Brutus cannot bring himself to confide in her. Perhaps he is trying to protect her, but it may also be that he in the inner recesses of his heart he is ashamed of what he has decided to do. Either way, Brutus appears as a man who, although he has decided on a course of action, remains divided against himself.
The scene also reveals the extent of Cassius's cunning. He thinks ahead, sees every possible obstacle, and tries to prevent each one.