Brutus and his friend Lucilius meet Pindarus, servant and friend to Cassius, with Titinius, another mutual buddy. (Wow, that's a lot of "us"es.)
Brutus says Cassius, who isn't there yet, has engaged in or overseen some shady business that makes Brutus wish they had never killed Caesar. Still, if Cassius is on his way, that's OK.
Lucilius admits, when asked by Brutus, that Cassius wasn't his usual friendly self. To Brutus, it sounds like the friendship is cooling.
Cassius's army will stay in Sardis (in what's now Turkey) that night, and the cavalry will arrive with Cassius.
Cassius enters and announces that Brutus has done him wrong. Brutus is shocked: how could he wrong someone who's like a brother to him? The two men are about to have a spat, and they agree it's best to do it privately rather than let the troops know they're fighting.
They order their armies to be moved away so they can go to Brutus's tent and argue in private, with Lucilius and Titinius guarding the door.