The play concerns the "seditious knight" Sir Timothy Treat-all and his Tory nephew Tom Wilding both vying for the affections of Charlot, the eponymous city (London) heiress. Treat-all keeps an open house for all of those who oppose the king, and he has disinherited Wilding.
Wilding launches a complex scheme to triumph over Treat-all. First, he introduces his mistress to Treat-all as Charlot, allowing Treat-all to woo her. This allows him to court the real Charlot himself, and it allows his mistress to move up to a wealthy knight. Then, during a staged entertainment, Wilding assumes a disguise and pretends to be a Polish nobleman. He offers Treat-all the throne of Poland. The greedy Treat-all accepts. Then, Wilding arranges for a burglary, where he and Treat-all both end up bound, and the burglars take all of Treat-all's papers. The burglars are Wilding's confidantes, and the papers contain evidence of Treat-all's treason. Wilding thereby marries Charlot, Treat-all marries Wilding's former mistress, and Treat-all is forced by blackmail to treat Wilding well and to leave him his estates