A Sicilian merchant has two sons, twins named Sosicles and Menaechmus. One day when his sons are seven years old, he takes Menaechmus with him on a business trip to Epidamnus. There Menaechmus is separated from his father and is lost in the city. He is found and adopted by another rich merchant and grows up in Epidamnus. The twins’ father is disconsolate over losing one of his sons and dies not long after the return to Syracuse. The remaining twin’s name is changed to Menaechmus to honor the missing brother.
Menaechmus I (the brothers will henceforth be referred to by Plautus as Menaechmus I, the brother lost in Epidamnus, and Menaechmus II, the Syracuse brother whose name was changed) lives a comfortable life. Though he despises his nagging wife, he enjoys visiting his prostitute Erotium, who lives next door. At the beginning of the play he is bragging to Erotium about the dress he stole from his wife to give to her. She is going to prepare a dinner for him and for Brush/Peniculus, a smarmy parasite who relies on Menaechmus I for good meals.
Menaechmus II and his slave Messenio arrive in Epidamnus. They have been traveling for six years, hoping to find Menaechmus II’s brother. Messenio is skeptical, thinking the brother must be dead, but Menaechmus II stubbornly insists he will not accept this until there is proof. Messenio warns him of this town, which is filled with harlots and swindlers.
The bulk of the play consists of numerous incidents of mistaken identity. Menaechmus II apparently does not think of the possibility that his twin might actually live here when he is constantly mistaken for someone he is not by the Wife, the Father, the Doctor, Erotium, and Brush. Menaechmus I, for his part, reaps the chaos stemming from conversations he does not remember happening when they were actually conducted by his brother.
When both brothers and Messenio are together in one place finally, Messenio points out the similarities and asks questions of Menaechmus I to ascertain his true identity. The brothers embrace and decide to live in Syracuse together. Messenio earns his freedom, first and illegitimately from Menaechmus I, whose life he saved when Father tried to commit him to Doctor’s care for his “madness,” and then second and legitimately from Menaechmus II, who is pleased with Messenio’s confirmation of Menaechmus I’s identity.