Octavius, Antony, Brutus, and Cassius all meet up before the battle starts where they disagree but agree that there will be no treaty and only one side will win.
Act Five Scene 3
Brutus sends Messala a message to have Cassius' army attack Octavius' because Brutus can see that he opposing army does not look ready and would be surprised.
Act Five Scene 4
Cassius and Titinius watch the battle from far and see that they are losing. Cassius sees a smoke coming from his tent and tells Titinius to go and see if it is from a friend or foe. Cassius tells Pindarus to watch Titinius and report back what is happening.
Act Five Scene 5
Pindarus mistakenly tells Cassius that Titinius was captured by the enemy. Cassius feels so guilty and realizes he won't have much more time to live himself and decides to kill himself instead of being killed. We later find out that Titinius was celebrating because Brutus was winning. When Titinius finds Cassius he kills himself as well.
Someone from Antony's army is sent to grab Brutus but they grab one of Brutus' loyal servants Lucilius. Brutus then hears of Cassius' death and plots to avenge him and he has a loyal friend kill him.
When Antony and Octavius found Brutus Antony said "He was the noblest Roman of them all". They also agreed to a proper funeral.