Octavius and Anthony confer on the plains of Philippi.
Octavius is surprised to see that Brutus and Cassius's army has come to meet them, especially since Antony thought the enemy would stay put. Antony thinks the enemy is fronting: clearly Brutus and Cassius mean to appear courageous and brave, but Antony can see right through that.
Antony and Octavius set up a battle plan and are met by Brutus and Cassius – each with his army behind him – for a pre-battle parley, or negotiation.
As Brutus tries to get them to reason (and maybe avoid the fight), Antony and Octavius bait him. They claim Brutus's words are no good when they're accompanied by bad strokes (of the sword). Antony's like, remember that time you cried "Long live! Hail Caesar!" while you stabbed him in the heart? This is a sore point for Brutus.
There's some more back and forth, and folks get testy. Finally Octavius draws his sword and says he won't put it back again until he's dead or Caesar's 33 wounds (not that anybody's counting) are avenged.
After more of this taunting, Antony and Octavius challenge Brutus and Cassius to meet them on the battlefield.
As Brutus talks with Lucilius privately, Cassius confides in Messala that it's his birthday. Though Cassius claims to be an Epicurean (meaning he doesn't believe in "signs and omens" mumbo jumbo), he's inclined to begin thinking differently after seeing something weird on his way from Sardis: two eagles swooped down from the sky, feeding out of the soldiers' hands. In the morning they were replaced by "ravens, crows, and kites" that spread like a shadow of death over the army. (Not a good sign.)
Messala tries to sway Cassius from the bad-omen talk, but Cassius brushes it off, saying he's still ready to face his peril.
Then Brutus and Cassius speak and agree to say goodbye to each other in a way that would be fitting if this were to be their last meeting ever.
Cassius asks Brutus what he'll do if things get bad, possibly hinting at suicide. Brutus points out that he condemned his father-in-law, Cato (who had fought on Pompey's side), for killing himself instead of giving himself over to Caesar. He's not sure what he'll do if they're defeated, as he finds suicide to be cowardly, especially if no one's really sure how things might turn out of they stay alive.
Cassius points out that if they lose, Brutus will be dragged through the streets of Rome.
Without explicitly saying he's decided to kill himself if they fail, Brutus declares that he'll never be taken to Rome in chains. Unless Brutus plans to catch the next plane to Vegas, we've just seen the great man commit himself to suicide.
Brutus and Cassius part nobly, saying that if they never see each other again, this was a good goodbye.
Brutus ends on a sort of Debbie Downer note, saying that although they don't know how, this day will come to an end. This is nice foreshadowing if you like that sort of "death is inevitable" talk.