The play opens on a crowded and noisy street in Rome as Julius Caesar returns from battle, where he stomped Pompey's sons into the ground.
FYI: Pompey is a guy who used to rule Rome with Caesar (they were called "tribunes"). After disagreeing with Caesar about how Rome should be run, Pompey was defeated in battle and assassinated. Just to be sure that Pompey's family and supporters couldn't come after him, Caesar chased Pompey's sons to Spain and defeated them in battle, too.
Murellus and Flavius, Roman tribunes who are friends of Brutus and Cassius, come upon a group of common people running about the street in their Sunday best when they should be working. The pair asks about the commoners' professions and what they're up to and finds out that they're on the way to celebrate and honor Julius Caesar.
Murellus and Flavius point out that rather than celebrate this victory, the people should get on their knees and pray against whatever evil will come from Caesar. They imply that Caesar will be tyrannical, having outlived the other two men who should share his power.
Before parting ways, Murellus and Flavius disperse the crowd and remove the party favors the people have left around Caesar's statue. They hope this will slow Caesar's roll a little bit as he prepares to overthrow the republic and make himself king.