In the world of Alfred Hitchcock, there is one golden rule: trust no-one.
This Dr Ben McKenna (Stewart) learns the hard way while tripping around Morocco with his wife Jo (Day) and young son Hank (Christopher Olsen).
Struggling to get to grips with local customs, the McKennas are first taken under the wing of French smoothie Louis Bernard (Daniel Gelin), then by the everso English Draytons (Bernard Miles and Brenda de Banzie).
The holiday takes a most peculiar turn when blacked-up Bernard dies in Ben's arms, but not before whispering that an assassination is about to take place in London. At the police station, the good doctor receives a call telling him to keep quiet or he'll never see his son again.
But isn't Hank with...? Nope, he's gone. Darn. Guess we're off to England then.
Hitch loved a goose chase, especially one combining his ability to make absolutely anyone look shifty with his passion for making life as uncomfortable as possible for his hero.
The moment when Jo's twittering friends turn up at their London hotel while Ben is making a crucial call is as squirm-inducing as knowing there's a madman on the other side of the shower curtain.
The suspense is leavened by Hitch's unique sense of humour, ranging from the mischievous (a sham vicar realises he has been rumbled while delivering a sermon on adversity) to the bizarre (a ruckus in a taxidermist's workshop).
Having his strings pulled by Hitchcock for the third time after Rope and Rear Window, Stewart is reliably fraught as the all-American foreigner but their most memorable collaboration was to follow two years later with Vertigo.
And while some may have viewed Doris Day's casting as a retired singer as little more than a gimmick, it's entirely relevant to the story. Once again, the biggest box office draw of the time proved that her range extended way beyond high Cs and popcorn flicks.
Moreover, the ever-hummable Whatever Will Be Will Be (Que Sera Sera) is probably the only Oscar-winning song to have any bearing on a movie's plot.
But it's an atypically drawn-out affair, Hitch dawdling within scenes and determined - or perhaps contractually obliged - to show off the widescreen wonder of VistaVision, particularly during the climax in the Royal Albert Hall (conducted by film composer Bernard Herrmann).
No matter. Even middling Hitchcock is a cut above most thrillers. And if you're lucky enough to catch it in high definition, you're in for a tangible treat.