Cassius and Titinius enter, with Cassius carrying a battle flag. Cassius tells Titinius that when his own flag-bearer started running away, Cassius killed him for his cowardice. Titinius says that Brutus gave his orders too soon, giving Antony’s men an opportunity to surround them. Pindarus enters, urging Cassius to quickly retreat—Antony’s forces are overrunning them. As Titinius rides off to scout out the situation further, Cassius reflects that on this, his birthday, his life has run its full course. Pindarus reports that Titinius has been taken captive by the enemy. Appealing to Pindarus’s personal loyalty to him, Cassius orders Pindarus to stab him to death. Pindarus does so, with the same sword Cassius used to stab Caesar. Then, in grief, he flees Rome forever.
Cassius’s army is being weakened both by cowardly deserters and by his overly-hasty actions against Octavius’s forces; now, Antony’s forces are surrounding them. Cassius also interprets the situation with undue haste, concluding that the battle is lost and accordingly committing suicide. While suicide is not out of line with Cassius's Epicurean beliefs, faith in omens is, and Cassius's hasty assessment of the battle's outcome is apparently influenced by his interpretation of the carrion birds he saw earlier.
Titinius and Messala enter. Messala tells Titinius that Octavius has been overthrown by Brutus, just as Cassius has been overthrown by Antony. Then they discover Cassius’s body on the ground. Titinius grieves his friend’s death: “the sun of Rome is set” because Cassius didn’t trust that Titinius would be successful in his scouting errand. Messala shares his grief, lamenting that despair moves people to irreparable error. Messala goes to give Brutus the sorrowful news while Titinius searches for Pindarus. However, alone with Cassius’s body, Titinius lays his victory garland on Cassius’s brow and then kills himself with Cassius’s sword.
The outcome of the battle is a draw—half of each army has been defeated. The omen Cassius saw was paradoxical. It influences him to believe the battle was completely lost when in fact it wasn't, so he kills himself, which causes his forces to lose the battle. Cassius would have been victorious if he hadn’t misconstrued the signs and despaired prematurely, setting off a chain of disasters.
Brutus, Messala, and several others enter. When they discover both Cassius’s and Titinius’s slain bodies, Brutus laments that Caesar’s ghost “walks abroad and turns our swords / In our own proper entrails,” and that Rome will never produce an equal to Cassius. He sends Cassius’s body outside the camp for burial and leads the others off for a second fight.
Brutus's comment reflects not so much a superstitious fear of Caesar's ghost as a growing belief that the conspirators’ deaths are deserved. In keeping with Brutus’s adherence to principle throughout the play, here he demonstrates his belief that situations that originate in wrong action can never be righted. Nevertheless, he plans to see the battle through beyond its present standstill.