The play revolves around Philocleon ("Friend of Cleon") and his son Bdelycleon ("Hater of Cleon"). Philocleon is addicted to the proceedings of the Athenian court; he spends all his time as a juror, judging others.
Bdelycleon wants to help his father; he locks him in the house, but Philocleon is determined to get out and go to the court. He attempts a few comical and unsuccessful escape attempts, including a play on one used by Odysseus on the Cyclops in the Odyssey. Philocleon's fellow jurors, a chorus dressed as wasps, come to his rescue. Bdelycleon and his servants brawl with the wasps, who, despite having actual stings, are soon outmatched. Afterwards, Bdelycleon engages his father in a debate, and soon proves to him and the chorus that by serving as jurors, they achieve nothing more than to serve the demagogues. The chorus defeated, Philocleon resigns to staying home.
To help his father with his addiction, Bdelycleon sets up a court in his own home for his father to preside over. For lack of anyone else to judge, Philocleon puts the family dog on trial; it ate a tasty piece of Sicilian cheese (apparently a reference to a recent trial in which Cleon accused the Athenian general Laches of accepting bribes from Sicilian enemies of Athens. Possibly also a mockery of Cleon's trial of Aristophanes himself, mocking Cleon, also known as 'the dog', for charging Aristophanes with what he saw as ridiculous charges).
In a farcical trial, Bdelycleon defends the dog and, when all else fails, a group of children dressed as the dog's puppies come on stage. Philocleon is unmoved, but Bdelycleon switches the ballot boxes on him, and he is tricked into voting "not guilty". When "all the votes" are counted, and the dog is acquitted, Philocleon faints; apparently he has never acquitted anyone before.
The two then go to a symposium (drinking-party). Furthering the theme of role-reversal, Bdelycleon teaches his father how to behave properly at the symposium; in this scene we see an (intentional) inconsistency of Bdelycleon as a character: he expresses disdain of Cleon's manipulation of the jurors, yet fawns upon Cleon and his lackeys in this 'upper-class' gathering.
Only the aftermath of the party is shown: Philocleon has gotten drunk, insulted almost everyone at the party, abducted a flute-girl, and has knocked over a bread-stall. Some of those he has wronged in the course of the night come to inform him of the suits they'll bring against him, but the unworried Philocleon mocks them. More significantly, he is happy; the chorus comments that this is quite an improvement for him. The play ends suitably absurdly, when Philocleon challenges three crabs (the sons of Carcinus, representing the tragedian playwrights of Athens) to a dance-off.