Rentadick is an English comedy where the individual parts are better than the whole.
Armitage (Donald Sinden), a chemicals manufacturer has two concerns: he suspects that his wife Utta (Julie Ege) is unfaithful and he needs to protect his new chemical formula from Japanese spies, led by Madame Greenfly (Tsai Chen). He retains security expert Major Upton (Ronald Fraser) to address both problems. Major Upton sets the virginal Hobbs (Richard Beckinsale) to spy on Utta. He sets his "Number 1", Hamilton (James Booth) to protect the industrial secrets. Unfortunately Hamilton, a rather dubious character with a sideline of kidnapping girls for shipment to the Middle East by Hussein (Michael Bentine), strikes a deal with Madame Greenfly to obtain the chemical formula for her. In this he is assisted by bumbling agency operatives Owltruss (John Wells - who is also credited with additional dialog for the film) and West (Kenneth Cope). Another agency operative, Miles Gannet (Richard Briers), manages to screw up both problems even more.
Some of the plot points have not aged well. The concept of female sex slaves for the Middle East makes one cringe. Similarly the stereotyping of the Japanese and Arab characters are inappropriate in a multicultural world. Anyone who is offended by these unfortunate cultural relics will likely find the film unacceptable and not amusing.
If you can look past those issues the film is moderately, but only moderately, amusing. The film was written by Monty Python's John Cleese and Graham Chapman. However, there is obviously something that happened with the film inasmuch as their screen credits were removed (presumably at their request). I would assume there was disagreement over the vision of the film inasmuch as the tone varies all over the place from drawing room to satire to absurdest. I was astounded to read on IMDb that the director, Jim Clark, was an Oscar winner (albeit for film editing on The Killing Fields, also nominated for The Mission). So there was some talent behind the camera.
All of the actors in front of the camera are quite talented (except perhaps for Julie Ege, who at least is quite decorative). Donald Sinden ,with eyebrows flying, takes the heroic British acting technique of "damn the material, full spreed ahead". Ronald Fraser comes off best with endearing comic mannerisms. James Booth, a good actor, suffers from a script that makes him a cartoon character throughout. Richard Briers is his usual fluttery nervousness. Richard Beckinsale (the father, by the way, of actress Kate Beckinsale and who died at the tragic age of 32) and Kenneth Cope have less to do. As for John Wells, well you either like in a mouse suit (don't ask) or you don't. The latter basically illustrates the issue of comedy in this film. Most of the sub-plot regarding Julie Ege plays well because the actors, most veterans of London's west end stage comedies, are used to the compromising positions, slamming doors, hiding etc. of this genre. However the film transforms into a more absurdest comedy as it moves towards resolution of the chemical formula plot line (which contains the more objectionable stereotyping noted above). Everyone suddenly has to become a cartoon and, while there is no British actor who isn't game, many can only go to louder exaggeration as a performance.
If you are offended by the sexual and racial concerns noted above you should avoid this film. If you can live them and the wildly uneven tone of the film then there is enough comedy and beloved actors to give this film at least one viewing.
P.S. if you want to see a movie written by John Cleese and Graham Chapman (albeit with Peter Cook) and featuring Ronald Fraser that really works I would strongly recommend the political satire "The Rise and Rise of Michael Rimmer" (regrettably very difficult to obtain in North America).