As the play begins, Hero is on the balcony of his house while Philia is at the window of Lycus's house. Senex comes out of his house and tells the Proteans (now dressed as slaves) that he and Domina are about to begin their journey to visit her mother in the country. Domina, who is clearly the ruler of the house, orders the slaves to fetch their bags and begins calling for Hysterium and Pseudolus. After confirming that Hysterium has prepared her usual potions, she tells him that while they are gone, Pseudolus is to be responsible for their son, Hero. Pseudolus is to take special care to keep him away from women, particularly those of the shameful house of Lycus. Domina sends Senex into the house to fetch a gift intended for Domina's mother, a bust of Domina. Hysterium is then appointed Slave-in-Chief, with instructions that his word is to be followed in their absence.
Hero drifts toward the house of Lycus after his parents leave; Hysterium goes into the house calling for Pseudolus. Hero tells the audience he's in love ("Love, I Hear"). The Proteans (now dressed as citizens) enter holding Pseudolus by the arms; they tell Hero that Pseudolus, masquerading as a citizen, has cheated them in a gambling game. At Hero's insistence, Pseudolus repays the citizens; he tells Hero he was only trying to gain enough money to buy his freedom. Hero tells Pseudolus he is in love with a girl from the house of Lycus. When Pseudolus explains to Hero he could never afford a courtesan from the house of Lycus, Hero announces he would give anything for this girl. Pseudolus asks for his freedom if he can arrange for Hero and his love to be together. Pseudolus, with encouragement from Hero, imagines all the things freedom would mean for him: the ability to vote and own property, as well as the responsibility for providing for himself ("Free").
Lycus appears from his house and berates a Protean (now dressed as a Eunuch) for fanning the courtesans while they are wet. Lycus announces he is off to the Senate to blackmail a Senator. Pseudolus takes Hero's money bag and jiggles it behind Lycus; the noise stops Lycus in his tracks. Pseudolus, after lying about where he got the money, asks to see the courtesans. Lycus summons the Eunuch to fetch them. They appear as Lycus describes their charms to Pseudolus, who looks to Hero to find the one he is in love with. After rejecting Tintinabula, Panacea, the Geminae and Vibrata, Pseudolus becomes captivated with Gymnasia, who enters performing a dance. Hero then has to remind Pseudolus of their agreement. As the courtesans go back in the house, Philia again appears at the window; Hero points her out to Pseudolus as the one they are looking for. Pseudolus asks Lycus about her, and Lycus explains she has already been sold to Miles Gloriosus, a great captain, who is coming later in the day to claim her. After Lycus tells Pseudolus Philia is from Crete, Pseudolus invents a story, telling Lycus Crete is suffering from a plague; Philia will soon die. Lycus tells him Philia seems healthy and spends the day smiling. Pseudolus says smiling is the signal death is near. Lycus, believing Pseudolus and afraid the plague is contagious, agrees to let Pseudolus look after Philia in Senex's house until the captain arrives.
Pseudolus goes to the harbor in search of a way out of their dilemma, leaving Philia and Hero together. After introducing themselves, Philia tells Hero she was never taught to add or spell or cook. Her only talent is that she is lovely. Hero quickly agrees with her ("Lovely"). After the song, they kiss and profess their love for each other. Hysterium enters and becomes very nervous when he sees them embracing; he gets more upset when he learns Philia is from the house of Lycus. Pseudolus enters just as Hysterium begins calling for him. After sending Philia and Hero off, Pseudolus first tries to tell Hysterium she is his daughter. When Hysterium refuses to believe that, Pseudolus tells him the truth about the soon-to-arrive captain. Hysterium announces he will tell the boy's parents, but changes his mind when Pseudolus reminds him that as Slave-in-Chief, Hysterium will be held responsible; Pseudolus also threatens to mention Hysterium's collection of erotic pottery. When the two young lovers return, Pseudolus announces his plan: they will go to the harbor with their belongings and take a ship to a far-off island where they can be free from all cares ("Pretty Little Picture"). They are about to go, but Philia reminds them all she is duty-bound to honor the contract with Miles Gloriosus. Pseudolus, realizing he will lose his chance for freedom if Philia does not run away with Hero, puts Philia in Senex's house to wait for the captain, whose arrival Pseudolus will announce with three knocks on the door.
Pseudolus decides upon a plan: he steals Hysterium's potion book in order to make a powerful sleeping potion for Philia; Pseudolus will tell Lycus and the captain she has died from the plague; Hero will then take the body away to the waiting boat. As Pseudolus goes off to find the sweat of a mare (a crucial ingredient for the potion), Senex returns; he has dropped Domina's bust and must now find a stonecutter to replace the broken-off nose. After calling out for Pseudolus, Senex goes to the door of his house and knocks three times. Philia opens the door, she assumes Senex he is the captain. She throws herself at him just as Pseudolus returns with a vial of mare's sweat. Philia exits back into the house. When Senex asks Pseudolus who she is, he tells him that she is the new maid. Senex and Pseudolus then discuss the joys of having a maid; they are eventually joined by Hysterium and Lycus ("Everybody Ought To Have a Maid").
Before Senex can enter his house to meet Philia, Pseudolus sprinkles some mare's sweat on Senex and convinces the old man he needs a bath after his long trip. Senex goes off to the empty house of Erronious to take his bath and wait for Philia. A very nervous Hysterium returns in time to see Senex enter Erronius's house. Hysterium tries to calm himself after Pseudolus goes off to make the sleeping potion ("I'm Calm"). He has almost calmed down when Senex yells for him; Hysterium runs into the center house.
Erronius returns home after his journey abroad searching for his children; he has very bad eyesight and mistakes Hysterium (who has just exited the house carrying a chicken) for a young woman carrying a baby. Senex (who is still waiting in Erronius's house for the arrival of Philia) begins to sing; Hysterium tells the startled Erronius his house is haunted. Erronius demands that Hysterium find a soothsayer; Pseudolus hears this and introduces himself to Erronius as a soothsayer. With Hysterium frantically gesturing behind Erronious, Pseudolus guesses the nature of Erronius's long trip abroad and tells him he can find his long-lost children. Erronius gives Pseudolus his ring, copies of which are worn by his children. Pseudolus then sends Erronius off to circle the entire city of Rome - with its seven hills - seven times; this will rid the house of ghosts.
Senex enters and sends Hysterium off to make a bath just as Hero enters looking for Philia. She steps onto the balcony, and Senex and Hero both realize that the other is watching her. This leads each of them, first, to deny the possibility that the other could be interested in her, then to find reasons why she would be more interested in the other ("Impossible"). Senex sends Hero to the public baths as Hysterium announces Senex's bath is ready.
Lycus enters; when he asks Pseudolus about Philia's condition, Pseudolus tells him it has clearly deteriorated; she has progressed from smiling to laughing. As Pseudolus is about to finish preparing the sleeping potion, a Protean (dressed as a soldier) arrives looking for the house of Marcus Lycus; he announces the imminent arrival of the captain, Miles Gloriosus, who is only half a league away. After the soldier leaves, Lycus and Pseudolus panic; Pseudolus decides he will tell the captain the center house, the house of Senex, belongs to Lycus. Lycus insists Pseudolus speak to the captain; Pseudolus agrees as Lycus runs off to move the courtesans from his own house to the center house. Two soldiers arrive carrying the contract between Lycus and the captain, and announce the captain is a quarter of a league away. Pseudolus, now claiming to be Lycus, tells them he intends to stand behind the contract. Hysterium enters calling Pseudolus; Pseudolus tells the soldiers his name is really Pseudolus Marcus Lycus and Hysterium is his eunuch. He tells Hysterium he deserted from the army long ago, and the soldiers are looking for him; Hysterium agrees to call him Lycus. Pseudolus instructs Hysterium to give Philia a few drops of the potion in a beaker of wine; at Pseudolus's command, Hysterium is to carry out her seemingly lifeless body. The soldiers leave after Pseudolus assures them the captain's bride will be ready when he arrives; Pseudolus tells Lycus to pose the courtesans informally in front of the center house. Everyone stops and watches as Erronius enters and exits after his first trip around the city.
Pseudolus sends Lycus into the house as a fanfare announcing the captain's arrival is heard. Pseudolus greets the captain and introduces himself as Lycus. Praising himself continuously, Miles Gloriosus demands to see his bride ("Bring Me My Bride"). Pseudolus calls for Philia to be brought out, but Hysterium enters and tells him Philia won't drink the wine for religious reasons. Pseudolus goes into the house with Hysterium to make Philia drink the wine. When Miles again demands his bride be brought out to him, Pseudolus exits the house and announces Philia has escaped. When Miles reminds Pseudolus (still pretending to be Lycus) the money for Philia has already been paid, Pseudolus drops his disguise and tells Miles Lycus will pay him back his money. Lycus and Hysterium enter and insist Pseudolus is Lycus. Miles Gloriosus, now in a rage, orders his soldiers to raze the center house. Miles threatens to kill Pseudolus, and Pseudolus, after begging time for one word, yells, "Intermission."
Prologus (this time played by Senex) welcomes the audience and recaps the action in Act One as the characters all enter and assume their positions from the end of Act One.
Miles, preparing to kill Pseudolus with his sword, orders a soldier to gag Pseudolus. Pseudolus escapes. He insists Philia must be nearby and he praises her beauty. Miles lowers his sword when Pseudolus tells him, "If you had been born a woman, you would have been she!" Miles orders Pseudolus, accompanied by his soldiers, to find Philia; she is to be brought to the center house where Miles and the rest of the soldiers will be waiting. Pseudolus tells Hysterium to bring Philia up to the roof of the house. Everyone stops and watches as Erronius arrives and exits after his second trip around the city. Miles gives Pseudolus one hour to bring Philia.
Senex, still in the house of Erronius and fresh from his bath, tells Hysterium to bring Philia to him; he also asks Hysterium to bring any left-over passion potion.
Pseudolus succeeds in losing the soldiers accompanying him and tells the audience his plan will still work if he can find a body. After he runs off to find the body-snatcher, Domina arrives. She runs into Hysterium, who is on his way to Senex with the passion potion. Domina tells Hysterium she still loves Senex despite her suspicion Senex is chasing other women ("That Dirty Old Man"). Domina, who is the daughter of a general, doesn't object when Hysterium tells her he has invited Miles and his men into the house. Miles, who thinks he is in the house of Lycus, believes Domina is an older courtesan.
Pseudolus returns and tells Hysterium the body-snatcher has died and someone has stolen the body. He pulls Hysterium into the house as Senex, still in Erronius's house, asks Philia, in the center house, to come to him; she is unseen by the captain, who has just appeared on the balcony of the center house. Philia runs into Hero on her way to Erronius's house; she tells him her revenge against being taken by the captain will be to think of Hero while making love to Miles ("That'll Show Him"). Pseudolus enters and sends Hero and Philia into the garden to avoid being seen by Miles; he also discovers Philia will go with Hero if Hero obtains the captain's contract. He then calls for Hysterium to come out of the house. Hysterium enters wearing a virgin's dress and wig. He complains about the costume as Pseudolus convinces him he must impersonate the captain's dead courtesan. When Hysterium continues to protest, Pseudolus tells him how pretty he'll look as a dead virgin ("Lovely - Reprise"); Pseudolus tells him to lie down and places Erronius's ring on Hysterium's finger.
Pseudolus calls out to the captain and his soldiers he has found the dead girl. Pseudolus offers to dispose of the body, but Miles insists on conducting a funeral service ("Funeral"). Miles wants to cremate the body, but Pseudolus convinces him the Gods would be unhappy. Miles then places the contract on the body. When Miles is about to kiss the body, Pseudolus tells him 'she' died from the plague in Crete. Miles, who has just returned from there, knows there is no plague; he leans over the body and declares "this girl is alive!" Hysterium jumps up and runs off, followed by Pseudolus and the soldiers.
Much running about and many scenes of mistaken identity follow: Senex mistakes Hysterium for Philia; Hysterium mistakes Domina for Philia; Pseudolus mistakes Domina for Hysterium. Finally, Domina runs into Senex, and Pseudolus, who has found the contract, gives it to Hero. Erronius enters after his third trip around the city and sees Hysterium, still in the virgin's clothes, running out of his house. Erronius, after mistaking Hysterium for the ghost who has been haunting his house, sees the ring on Hysterium's finger, and thinks Hysterium is his long-lost daughter. Senex arrives and announces Hysterium is his new maid. Miles enters and claims Hysterium as his virgin. Hysterium's wig falls off and everyone's identity is revealed. An angry Miles is again about to kill Pseudolus, who asks to be allowed to kill himself. He tells Hysterium to bring him what he thinks is the sleeping potion; Hysterium, however, brings him the passion potion. Lycus enters with Philia and presents her to Miles. When Erronious mentions his confusion about the ring on Hysterium's finger, it is revealed Miles and Philia each have identical rings - they are brother and sister and Erronious is their father: Erronious is finally reunited with his children. Hero tells his parents of his wish to marry Philia. The entire ensemble joins Pseudolus as he explains everyone has gotten what they have been looking for ("Comedy Tonight - Reprise").