It seems that Fu Manchu survived the previous film again (no surprises there) and has retreated to his native China for this third film in the series, where he is busy cooking up his next diabolical plot with his Daughter Lin Tang (with Christopher Lee and Tsai Chin reprising their roles from the previous films).
This time, his primary goal is to eliminate his nemesis, Nayland Smith of Scotland Yard (with Douglas Wilmer returning from the previous film), in which he intends to disgrace him, before having him killed.
So when Nayland Smith seems to suffer some sort of breakdown, then murders his housekeeper, no-one, including his friend and assistant Dr Petrie (Howard Marion-Crawford), can believe it's actually him.
So when they discover that Fu-Manchu is somehow behind this, a group of intrepid Interpol agents set out to his hideout to put an end to his nefarious activities and find out what he's done to Nayland Smith. Leading to another series of fights, gun battles and rather nasty death scenes.
Filmed at the legendary Shaw Brothers studios in Hong Kong, this one was clearly aimed at a slightly more mature audience this time. Featuring more graphic death scenes and stronger language, which is arguably the best of the first 3 films.