The surrealistic opening sequence, featuring a WW2 calendar as written "by A. Hitler", should be indication enough that Once Upon a Honeymoon is no ordinary lighthearted romantic trifle. Ginger Rogers plays Katie, an American chorus girl who seeks to better herself by marrying titled European Baron von Luber (Walter Slezak), despite the warnings of reporter Pat (Cary Grant). Katie thinks Pat is just jealous, but both he and the audience are aware that Von Luber is secretly a high-ranking Nazi, whose "unofficial" visits to Czechoslovakia, Poland and France precipitate the German invasions of those countries. When Katie wises up, she agrees to help counterespionage agent LeBlanc (Albert Dekker) in his efforts to stop Von Luber before he can reach New York-and along the way, she falls in love with the ubiquitous Pat. The bizarre ending, in which one of the main characters is casually murdered, is played for laughs, as if WW2 is merely fodder for a screwball comedy. In the film's most unsettling scene, Katie and Pat, mistaken for Jews, are briefly interred in a Polish concentration camp; their outrage over this treatment seems to be founded not on Germany's crimes against humanity, but over the fact that the Gestapo would have the audacity to incarcerate two non-Jewish Americans! A curious and often tasteless misfire from producer-director Leo McCarey, One Upon a Honeymoon is an undeniably fascinating historical artifact.
~ Hal Erickson, All Movie Guide