The character of Fu Manchu was a supervillain created by British author Sax Rohmer in the 1910s. The criminal mastermind proved popular and appeared in a number of books, radio shows, comics, and movies. A series of five films starring Christopher Lee in the titular role were made between 1965 and 1969. I don't know how good the first four were, but the fifth made for a brutal MST3K subject
Our movie opens in tropical waters where the evil mastermind is using his freezing device to sink the Titanic. Well, it's not supposed to be the Titanic, it's supposed to be a generic luxury liner, but the stock footage comes from well known film A Night to Remember (1958). (The warmly dressed victims make the claim that the sinking has occurred in tropical water laughable.) Unfortunately for our villain, the device burns itself out and he needs the help of the brilliant Dr. Heracles to fulfill his plot of freezing the world's oceans.
Confident that he'll be able to carry out his threat, Fu Manchu starts to make demands of the world's governments. Nayland Smith of Scotland Yard is soon put on the case. Unfortunately, the designated hero is such a non-entity in this film that the plot could very nearly be summarized without actually mentioning him. Smith makes Diamond Head look competent and effective by comparison.
Anyway, since Heracles' technique requires crystals of opium, Fu Manchu decides to take over the governor's castle in Istanbul, which happens to have a huge opium reserve. To pull this off, he enlists the help of opium dealer Omar Pasha. However, once their combined forces have successfully raided the castle, Fu Manchu reneges on the deal and kills Pasha's men while imprisoning Pasha's assistant, Lisa. (Lesson number 1: never trust the word of a supervillain.)
Now in control of the castle and its opium, Fu Manchu runs into another problem; Dr. Heracles is dying of a heart condition. Heracles' cardiologist, Dr. Kestler, and his assistant Ingrid are promptly kidnapped with little resistance (while Smith is in the house, no less). The two are forced to perform a heart transplant... in a fully equipped operating room... in a castle in Istanbul. (The filmmakers' contempt for logic or sense is astounding.) Meanwhile, Smith somehow figures out that Fu Manchu is in Turkey and makes an alliance with a vengeful Pasha to try to get into the castle. Pasha meets with Fu Manchu and offers to hand over Smith in exchange for Lisa (it's never clear if this is part of their plan or if he's betraying Smith) and is killed for his trouble.
After recovering from his surgery, Heracles refuses to give up his formula... and in the very next scene Fu Manchu has somehow gotten it out of him. (Any review of this movie is bound to use the word "somehow" repeatedly.) In no time at all the formula has been weaponized; all the villain has to do is introduce it into a canal below the castle which leads to the Bosporus strait. Apparently the resulting chain reaction will freeze the strait and all contiguous bodies of water. Or something. I'm kind of guessing here since the filmmakers didn't really think we needed to be let in on the plot.
The audience is then treated to one of the most pointless, disjointed, and nonsensical climaxes ever. Smith infiltrates the castle with little problem and then runs around taking out Fu Manchu's men in poorly choreographed fight scenes. Kestler and Ingrid (who have gotten more screen time than our supposed hero) manage to escape. Smith rescues Dr. Heracles and Lisa, who has done absolutely nothing in the film since her capture and serves no function in the rest of the plot. Once Heracles is safe, Lisa returns to the castle to save Pasha (I have no idea how she knew he was ever in the castle) and is drowned in the canal. The freezing formula is released into the canal but the chain reaction fails for some reason. That's right, the world is saved because the villain's doomsday weapon simply doesn't work. In fact, all the hero ever manages to do is save Dr. Heracles since Lisa dies anyway and Kestler and Ingrid are smart enough to save themselves. Smith might as well have stayed home. Finally, just because, the weapon backfires, blows up the castle, and we're left with Fu Manchu's face superimposed over the dust cloud and his "ominous" declaration that he'll be back. (Actually no, he won't. This was the last of Christopher Lee's Fu Manchu movies.)
The Castle of Fu Manchu may very well be one of the ten worst movies I've ever seen. The hero is bland and worthless. Christopher Lee's Fu Manchu spends most of the movie looking bored (it could be the makeup that Lee wears) and is incapable of pulling off much in the way of super-villainy even when his arch-nemesis is incompetent. The plot is immensely hard to follow and the quality of the camerawork and the film stock is poor. The movie is so bad that it repeatedly reduces Joel and the 'bots to tears.