The main characters of Phormio are Phormio, a crafty lawyer, and the cousins Antipho and Phaedria, two young men whose fathers, Demipho and Chremes, are brothers. The young cousins are the suitors of Phanium and Pamphilia. The play takes place in Athens, and the plot revolves around their courtship and marriage, including the fathers’ disapproval and manipulation, and Phormio’s efforts to aid the hapless young lovers.

The young men are of good families, as is Antipho’s beloved Phanium, but she has no money. When his father finds out they had secretly married, he embroils Phormio in a plot to lure her away and save his son so he can marry a suitable (wealthy) woman.

Phaedria’s beloved, Pamphilia, is also considered unsuitable, as she is a slave. Phormio applies himself to making this union a success and thwarting the fathers.

The true identity of some characters forms part of the plot’s many twists and turns. Chremes had committed a youthful indiscretion in Lemnos, which led to the birth of a daughter. It turns out that she is none other than Phanium, whom Antipho has just married.

Nausistrata is Chremes’s wife. Phormio’s disclosure of her husband’s daughter’s existence sets in motion the play’s resolution, with the two brothers supporting their children’s marriage. (Cousin marriages were then considered a good thing.)