The mob of plebeians encounter Cinna the Poet in the street. They interrogate him about his identity, and start to attack him when they learn his name, thinking him to be Cinna the conspirator. When he tells them that he is Cinna the Poet, they attack him anyway, for being a bad poet.
This is the first (and last) comic scene since the punning exchange that began the play. It breaks the tension before Act 4, and bolsters the image of the plebeians as a mindless herd.