This is easily the best of producer Harry Alan Towers' '60s reboot of Sax Rohmer's Fu Manchu series. Christopher Lee is a perfect fit as the imperious, degenerate supervillain, almost matched by Nigel Green in a rare starring role as the crime fighting Sinologist, Nayland Smith. Essentially, it's a Holmes-Moriarty story.
There are echos of the James Bond franchise too, though arguably as Dr. Fu Manchu turned to crime in 1911, the influence may be the other way. The evil Chinese terrorist intends to kill everyone on Earth with poisonous opium, for reasons which are never made clear. It's just a hook for some hokey dialogue and sadistic mayhem.
The face of Fu Manchu is actually immobile, given Lee's heavy yellowface makeup... There's a decent budget and some nice locations, with Dublin standing in for London. And China! The relentlessly overcast weather looms gloomily over the action. The period reconstruction is fair. This was a co-production so there's an odd supporting cast of German actors.
The narrative stirs up the usual formula of an exotic adventure driven by a cruel megalomaniac. Which is amusing, without being exciting. But Fu Manchu adds some cultural resonance. This sort of story was realised better in precode Hollywood. They did arcane melodrama better back then, and it's easier to buy into the foolishness.