Every now and then Julie Andrews tried to "shake up her image" - she did it in her second film, The Americanisation of Emily, then in Torn Curtain, then here, then in The Tamarind Seed, then SOB. Never did anyone particularly care - while she's a perfect singing nanny, she's not remotely interesting as a sexy seductress. Here she does a strip tease (down to her underwear only, admittedly), is a spy for the Germans in World War One, seduces Rock Hudson, kisses him naked while in the shower... but she's still just good old Julie Andrews.
It's a tribute to the filmmakers that this cost so much money. There are some snazzy enough production numbers but not like teams of extras - some songs in a tavern, and a music hall (the old WWI songs are sung beautifully by Julie A, incidentally). There are some airplane stunts, which are impressive enough (b-plane action) but mostly dogfights between one or two planes - nothing on the scope of Hell's Angels or something like that. (And the plane stuff could have even be cut out, too - the movie could have worked set all indoors. Which is why it's hard to believe director Blake Edwards' claim that the movie cost an extra $5 million due to bad Irish weather.)
It's not a particularly interesting story. The set up is okay, kind of like Ninotchka - a German spy is turned via love for an Allied person she sets out to seduce, in this case Rock Hudson. Hudson is dashing and handsome enough. But Lili isn't much of a character - in Ninotchka Garbo's character was clear, she was dour and humourless and loved communism, until converted via love and champagne to the delights of capitalism. Here Andrews' Lili is a feisty singer and super spy who works for the Germans because... um... anyway, she does, and she falls for Hudson because, um, I guess he's hot, and it's love (they have okay chemistry but not treacherous-inducing, I would have thought), so she changes sides, and gets jealous at the thought of him with someone else.
This isn't offensively bad or anything, I urge all Julie Andrews fans to see it, they'll get a lot out of it - it's just not particularly well done. There are some good things like the production design, and a strong support cast of character actors, plus pleasing photography.