Set on a street in Ancient Rome the plot for 'A Funny Thing Happened On The Way To The Forum' revolves around three houses - the house of Senex who lives there alongside his wife Domina, son Hero and their two slaves including the musicals main character Pseudolus, a brothel owned by Marcus Lycus, and the house of Erronius, who for much of the play is absent searching for his long lost children who had been stolen by pirates in their infancy.

Pseudolus is desperate to gain his freedom and so the musical begins when he agrees to help Hero as part of a deal that will see him win his freedom. Hero has fallen in love with Marcus Lycus' latest purchase - the virgin Philia, and wishes Pseudolus to help him woo her and arrange for them to run away together. Unfortunately Philia has already been sold to the warrior Miles Gloriosus and despite falling in love with Hero insists that she upholds the contract.

Pseudolus cleverly manages to convince Marcus Lycus that Philia is actually suffering from a Cretan plague and so is able to hide her in the house of Senex whilst he goes in search of ingredients to create a sleeping potion that will allow them to escape with her. However, the plot thickens when Senex unexpectedly returns home and there is some confusion where Philia believes that he is her buyer Miles Gloriosus and offers herself to him. In order to 'prepare' himself properly he decides to go bathe in the empty house owned by Erronius, yet suddenly Erronius himself returns having finally given up on the search for his lost children. In a panic and in an attempt to disguise the fact that his master is bathing in the house, the other slave Hysterium manages to convince Erronius that his house is, in fact, haunted and that he must fetch a soothsayer at once. Pseudolus steps in at once as a pretend soothsayer and in order to keep the old man out the way convinces him that he must travel around the seven hills of Rome seven times.

When Miles Gloriosus turns up to claim Philia, Pseudolus tells Marcus Lycus that she has escaped, however he graciously offers to step in and pretend that he is Marcus Lycus to Miles Gloriosus so that the real Marcus Lycus does not need to face his wrath. He tells the enraged Miles Gloriosus that he, Marcus Lycus, will find Philia and eventually manages to give the guards that Miles Glorisus had insisted on accompanying him the slip.

Senex' wife Domina now returns after having a strong feeling that Senex will be up to no good and dresses up in virginal garb similar to Philia, complete with veil. Hysterium is also convinced to dress up in drag and pretend to be the dead Philia - having finally succumbed to the 'plague', however the ruse is broken as Miles Gloriosus has himself just returned from Crete and can confirm that there is no plague.

A farcical chase now occurs with Senex and Miles Gloriosus now chasing the three identically dressed 'Philia's', and to make matters worse the other courtesans from the house of Marcus Lycus have now escaped also - causing pandemonium across the stage.

The play finally ends when the plot is finally found out, but just as Pseudolus looks to be in deep trouble Erronius arrives having completed his trek across the hills of Rome. He instantly recognises the two rings that both Philia and Miles Gloriosus wear and it turns out that they are both his long lost children. A happy ending is had by all as Hero marries Philia, Miles Gloriosus is placated with twin courtesans, Pseudolus is granted his freedom and a courtesan of his own and Erronius is reunited with his two children. However Senex is still left with his shrewish wife Domina.