So Long at the Fair (1950). Jean Simmons is an adorable young lady at the 1896 Paris Exposition. She is absolutely charming and, until the plot gets weeeeeeeird, she totally out-Audrey Hepburns Audrey Hepburn, and it was done before Audrey Hepburn was even Audrey Hepburn. David Tomlinson plays her brother who makes a sudden disappearance in the middle of Act 1. He disappears so well, he takes his hotel room with him. You'd probably remember Mr. Tomlinson best as Mr. Banks in Mary Poppins. Honor Blackman is also in it, but she's got a supporting role and in no way resembles Miss Pussy Galore.
The plot comes right out of a popular urban legend. I never realized this particular one was so old.
BONUS FUN AWKWARD MOMENT!
YOUNG MAN: My, but that young woman you're with is beautiful. TOMLINSON: That young woman is my sister. YOUNG MAN: Oh! Terrible luck, then.