The play begins with a strange scene—a large net has been spread over a house, the entry is barricaded and two slaves, Xanthias and Sosias, are sleeping in the street outside. A third man is positioned at the top of an exterior wall with a view into the inner courtyard but he too is asleep. The two slaves wake and we learn from their banter that they are keeping guard over a "monster." The man asleep above them is their master and the monster is his father—he has an unusual disease. Xanthias and Sosias challenge the audience to guess the nature of the disease. Addictions to gambling, drink and good times are suggested but they are all wrong—the father is addicted to the law court: he is a "phileliastes" () or a "trialophile." The man's name is "Philocleon" (which suggests that he might be addicted to Cleon), and his son's name is the very opposite of this—"Bdelycleon". The symptoms of the old man's addiction include irregular sleep, obsessional thinking, paranoia, poor hygiene and hoarding. Counselling, medical treatment and travel have all failed to solve the problem, and now his son has turned the house into a prison to keep the old man away from the law courts.