This was Christopher Lee's fifth outing as the Oriental arch-villain for producer Harry Alan Towers and the second one from director Jess Franco. This time around Fu and his faithful daughter Lin Tang (Tsai Chin) have a device that will freeze solid the worlds oceans. This is shown in the opening scenes of the film as he uses to sink an ocean liner in the "tropics" (which is strangely populated with passengers bundled up for winter), as this orgy of stock footage comes from 1958's Titanic opus A Night To Remember. This pretty much gives a big hint to the production values (and logic) of the rest of the movie. After discovering his new "ocean freezing" invention needs some tweaking to prefect it, he kidnaps a Dr. Heracles.
The Dr. informs him that massive amounts of opium crystals are needed to make it work and with this in mind he takes over the governor's castle in Istanbul with the help of opium dealer Omar Pasha (along with his assistant Lisa - Rosalba Neri), because as it turns out the governor's castle has a massive supply of opium stashed there. Opps ! - seems that the Doc has a bad heart, so next Fu kidnaps a heart surgeon and as the castle happens to have a fully equipped operating theatre (along with the tons of opium) this is no problem. After taking over the castle he double crosses Omar and throws Lisa in the dungeon. Soon Fu Manchu's nemesis Nayland Smith (Richard Green) shows up, beats up a few of the guards in some really badly choreographed fight scenes and by doing absolutely nothing at all saves the day in the end. "I think" & "somewhere" would seem to be the two most common phrases you'd use in describing the plot.
A typical Jess Franco "I don't really care about this one" production with a very ludicrous plot and huge jumps in logic (lots of "Hey, how'd that happen ??"). Rosalba's presence (and Chris Lee) was the only thing that helped me make it thru this, as it is an endurance test. The plot clunks & drags along and when something needs to happen, it usually just happens - with no explanation. Fu Manchu needs to get the secret formula from Dr. Heracles and he won't give it up ? Hey, no problem as Fu has it in the very next scene - as things just sorta happen "somewhere". His laboratory consists of what looks one of those chemistry sets you use to be able to order out of the Sears catalog placed upon a wooden table and the dungeon consists of paper mache rocks and lots of psychedelic lighting (to most likely help HIDE the paper mache rocks).
Rosalba (3rd billed here in the secondary credits) shows up for about the first twenty minutes then disappears until the climax as she escapes from the dungeon and attempts to rescue Omar. For some strange reason (I'm sure known only to Jess) he dresses her a series of really ugly men's clothing - including a stripped suit and fez (??) and almost seems to go out of his way to make her seem unattractive (which is a real challenge). Not the best movie on her resume, but hey its Rosalba, so here you have it.