That night there is a fearsome thunderstorm. Meeting Cicero in a Roman street, Casca tells of strange things he has seen and heard about that night. Women swear they saw a hundred men, covered in fire, walking up and down the streets; he himself saw a lion at the capital and a man with his hand on fire that flamed like twenty torches without burning him. These strange events mean that something ominous is about to take place.
Cassius enters. He has been wandering the streets during the storm. He tells Casca that the unnatural disturbances are a warning that something is badly wrong with Roman society. He says that he could name a man who is every bit as ominous as the signs in the stormy night. Casca knows that he refers to Caesar, and Cassius says that Romans are feeble because they put up with his tyranny.
Casca believes that the following day, the Roman Senate will offer Caesar a crown. Cassius boasts that he has the power, and will use it, to overthrow tyranny. Casca agrees, and Cassius continues, saying that the only reason Caesar acts like a tyrant is because he knows Romans are weak and will not resist. Casca and Cassius agree to spearhead the plot to assassinate Caesar.
Another conspirator, Cinna, enters. Cassius gives him the letters he has written and tells Cinna to throw them into Brutus's house. After doing this he is to meet Cassius and some other conspirators at Pompey's Porch, which is the portico of a theatre built by Pompey. Cassius tells Casca that Brutus is three-quarters won over to their cause, and on their next meeting he will embrace it fully.
As often occurs in Shakespeare's plays (King Lear for example), a disturbance in the macrocosm (the violent storm) indicates a disturbance in the microcosm, the world of man and his society. The scene also reveals more of the central characters: Cassius's ambition for power and Brutus's decency and reputation as a man of honor. This is why the conspirators need Brutus on their side: it will make their cause seem respectable.