The World of Henry Orient (1964)

While trying to seduce a married woman (Paula Prentiss), egotistical concert pianist Henry Orient (Peter Sellers) is interrupted by two precocious schoolgirls, Valerie (Tippy Walker) and Marian (Merrie Spaeth). At first he thinks nothing of it, but as he sees them more often, he becomes convinced they're spies for his lover's husband. The girls, meanwhile, have found out who he is, and Valerie insists she's in love. She starts a scrapbook, which her mother finds and, of course, misconstrues.