Welsh librarian John Lewis (Peter Sellers), unhappily married to Jean Lewis (Virginia Maskell), falls in love with the glamorous Elizabeth Gruffydd Williams (Mai Zetterling). Zetterling is likewise saddled with a dull spouse, wealthy Vernon Gruffyd-Williams (Raymond Huntley). Finding themselves to be kindred spirits, Sellers and Zetterling plan an illicit affair. Alas, none of their carefully calculated schemes for a romantic tryst come to fruition thanks to a series of comic (but utterly credible) complications. John ultimately concludes that adultery simply isn't worth the bother.