The root of Cassius and Brutus' argument comes out: Brutus has condemned a man, Lucius Pella, for taking bribes from the Sardians. Cassius wrote a letter saying Pella shouldn't be punished, but Brutus ignored it. He accuses Cassius of being dishonorable for suggesting they let bribery slide.
Cassius resents being called greedy, but Brutus gets to the heart of the matter: they all killed Caesar for justice's sake, but when they start getting involved in petty robbery, it compromises their honor and calls into question their noble motives for killing Caesar.
Cassius and Brutus then argue, and Brutus is all "I don't even know who you are anymore." Brutus tells Cassius to get out of his sight, which doesn't go over well, and the two start threatening each other.
Brutus brings up an old problem: he had asked Cassius to send gold to pay his soldiers, but Cassius denied him, which was not cool. Cassius claims he didn't deny Brutus; it must've been some bad messenger's fault. Still, Brutus should be a good friend, Cassius says, and ignore his faults. That's what friends do.
Things come to a head when Cassius offers Brutus his blade and naked chest. Cassius points out that Brutus stabbed Caesar out of love, which is more than Cassius is getting from Brutus right now.
With the offer of murder on the table, they both realize they're being a bit moody and melodramatic. They agree that Cassius is showing his mother's temper again. From now on they'll be friends and not get angry at each other.
As they step out of the tent, they find a poet waiting to tell them they should be friends. It's really nice of the poet to be so concerned. They laugh at him and send him off, then they direct Lucilius and Titinius to get their armies ready to lodge for the night.
Then the big news about what put Brutus in such a bad mood comes out. Portia, Brutus's loving wife, was driven to grief by his flight from Rome and by Antony and Octavius's growing strength. Long story short, she has killed herself by swallowing coals. (Ouch.)
After he tells all this to Cassius, Brutus gets some wine and aims to drink the pain away, saying they should speak no more of his dead wife.
Messala and Titinius come in, and though Cassius would like to dwell on Portia's death a bit, Brutus is all business.
They've learned that Octavius and Antony have decreed that a hundred senators must die in Rome. Both men are now on their way to Philippi. Brutus says he's only heard the names of seventy senators, and that Cicero is one of them. Messala then pipes up that Cicero is dead, and tries to skirt around the issue of Portia's death with Brutus.
Brutus is less hurt than anyone expected him to be. He says Portia had to die only once, and he can bear that death.
The talk then turns to beating their enemies at Philippi. Cassius thinks it's better for them to sit tight until Antony and Octavius wear out their own armies with travel. That way Brutus and Cassius' army will still be fresh to fight.
Brutus points out, though, that the enemy army might gather strength as it goes. Because more and more men between Rome and Philippi don't support Brutus and Cassius, they might be willing to join Antony and Octavius' forces. Brutus thinks his and Cassius' army is at its peak right now. They'll only get weaker, so it's better to act right away.
They all agree to go to Philippi and meet Antony and Octavius' army.
Everyone decides to get a little sleep. They all say their "goodnights" to one another, and Brutus has Lucius call in some soldiers to sleep in his tent just in case he needs them to take messages to Cassius in the night.
Brutus is apparently pretty keyed up. He asks Lucius to play him a tune on his instrument, even though Lucius is sleepy.
Lucius plays, but falls asleep mid-song. With everyone else asleep, Brutus picks up his book to read.
Just then Caesar's ghost shows up, claiming he is "thy evil spirit, Brutus." Brutus is a bit shaken, and the ghost explains that he'll see him again at Philippi. Brutus is all "see you then, I guess."
After the ghost disappears, Brutus wakes the men who've been sleeping in his tent. None of them saw the ghost.
Brutus has one of the men tell Cassius to send his army off early in the morning; Brutus' army will follow. It seems Caesar's ghost has only cemented Brutus' willingness to meet his fate, whatever it be.