During the gloomy days of World War I, audiences, including servicemen on leave, wanted light and uplifting entertainment, and they flocked to theatres to see lighthearted musical comedies, a number of which broke box-office records. It opened at the Adelphi Theatre in London on 14 September 1917, and ran for 801 performances – one of the longest runs of any musical theatre piece up to that time.
Mrs. Millicent Meebles, a remarried widow, habitually understates the age of her son, Hughie Cavanaugh, to make herself appear several years younger. Consequently, the 19-year-old lad is taken for a 14-year-old boy and appears to be precocious. Hughie, that "Sporty Boy", flirts with astonishing skill with his beautiful music teacher.
Hughie also precociously introduces his step-father, Mr. Meebles, a magistrate, to the nightclub in the Cosmos Hotel. They are followed by Millicent, who wishes to warn a friend not to divulge the boy's true age. When the nightclub is raided by the police, Mr. Meebles escapes without capture, having a comically difficult time.
Mr. Meebles arrives at court just in time to hear the case that results from the raid and must, as magistrate, sentence his wife and sister-in-law, Diana Fairlie (whose lover is Albany Pope), to incarceration. Fortunately, a fellow magistrate, Mr. Burridge comes to the ladies' rescue by over-ruling the sentence on a technicality. At home, Millicent confesses that she has exaggerated Hughie's age. All is forgiven.