Phormio is a comedy that centers on the romantic prospects of two young men, Antipho and Phaedria, and their beloveds. One couple, Antipho and Phanium, has secretly married while his father, Demipho, is out of town. The other father opposes Phaedria's chosen girlfriend, Pamphilia, because she is a slave. Phaedria must try to buy her. The plot has some unexpected developments before the lovers' plights are happily resolved.

Phormio, the title character, is an attorney and a specialist in fixing complicated situations. He apparently agrees to work for Demipho in breaking up his son's marriage, but instead he takes the fee he was paid and uses the money to help the other young lovers.

The plot further thickens when it is revealed that Chremes had a long lost daughter, and coincidentally she is none other than Phanium. Now he can gladly endorse his nephew's choice of bride and be reunited with his daughter.