The year is 1912, and it's time for a summer garden party at the estate of Lord and Lady Roper. Guests are coming in from all over England, and all the talk is about Lord Roper's dashing young nephew, John Tripper. The irrepressible raconteur delights all the ladies not only with his surprising skill in the kitchen, but also with his tales of his daring life. Soldier of fortune, gentleman thief... Is he telling the truth, or merely a master liar? Matters are desperately complicated for John when Lord Roper discovers him in the chambermaid's bedroom, and, thinking fast, John claims that he is, in fact, a homosexual. Not wishing to be scandalized, Lord Roper agrees to keep the matter silent, and John thinks he's gotten away with it. But when his parents disinherit him, he is left with no other home but the Roper estate, and for once he must live with one of his lies. Soon, misunderstanding piles upon misinterpretation as John must keep track of all the many and varied lies he's told... and to whom! Drawn into this are Lord Roper's daughters, the brainy Jeanette and the not-so-brainy Christine, enlisted by John to help in his escapades; and the estate's gardener, Furleigh, enlisted by Lord Roper to uncover the truth.
The advantages to this time period are clear. The importance of politeness and propriety make the bawdy misinterpretations of innocent events and the wacky explanations of bawdy events funnier by a factor of about a thousand. Also, the setting allows for such a broad range of personalities - a cockney chef, a Great White Hunter, Agatha Christie, Bertie Wooster... the possibilities are endless. Making almost everyone relatives was for the convenience of the plot, but I think it can add to the humor. For further reference on this type of plot in this time, see The Importance of Being Earnest, the non-depressing parts of Atonement, and the Doctor Who episode "The Unicorn and the Wasp", only on that last one you have to add in sex jokes and take out the giant murderous alien insect.