If you're in the mood for a cat burglar movie this will hit the spot because its got every cliche/trope of the genre your heart could desire: George Hamilton in an all black outfit climbing down a wall during the opening credits, a wise old protege (Joseph Cotten), a beautiful rival female cat burglar (Maria Laforet) and her protege (Maurice Evans), a dogged detective to bring him down, a key jewel, European locations (ski fields, the beach), a big final heist, Hamilton has a trapeze in his apartment that he uses to work out, lots of fancy outfits, Hamilton is called "Ace", Cotten has a secret involving Laforet.
I guess it does lack a beautiful heiress whose jewels are coveted by the burglar. However it does have a series of cameos by actresses playing themselves - Zsa Zsa Gabor, Carroll Baker, Lili Palmer.
It could have done with a third act betrayal and a tightened running time - it feels flabby. The last ten or so minutes drag especially.
George Hamilton tries but is a long way from Cary Grant - a lot of actors got roles by being newer, younger versions of old stars (eg Matthew McConaughey was the new Tom Cruise, Julia Ormond was the new Audrey Hepburn) and only survived if they carved out their own niche; Hamilton would but the niche was as a jokey suntanned playboy, not Cary Grant. Marie Laforet is beautiful but indistinguishable from numerous European actresses who popped up in films around this time. It's a pleasure seeing Cotten - a movie about him teaching Hamilton (the back story to this) would have been more interesting than what's going on; their relationship is reminiscent of Bob Wagner and Fred Astaire in It Takes a Thief. Maurice Evans is alright - might have been nicer if they'd cast some old time movie star to square off against Cotten.