Prominent attorney Walter Pidgeon takes a murder case pro bono, wins an acquittal and discovers that his client (Keefe Braselle) was not only guilty but part of an extortion ring reaching to the highest eschelons of the city. Panged by his own complicity, he undertakes an investigation, stumbles onto the identity of the "unknown man" who heads the syndicate, and murders him.
The ironies engage when Braselle is charged with this second murder and Pidgeon must defend him by pointing to the existence of another "unknown man" -- himself. Though somewhat short of urban grit and long on rhetoric, the Unknown Man belongs to the noir cycle less by style or structure than by its acknowledgement of the pervasive corruption of American municipal politics that came to light in the postwar years.

One of the weaker Carry On adventures sees Sid James as the head of a crime gang stealing contraceptive pills. The fourth of the series to be hospital-based, it's possibly the least of the genre. There's a curiously flat feel throughout, with all seemingly squandered on below-par material. This is far from the late-70s nadir, but Williams, James, Bresslaw, Maynard et al. are all class performers yet not given the backing of a script equal to their ability.
Most of the gags are onrunning, rather than episodic as Carry Ons usually are. So that instead of the traditional hit and miss ratio, if you don't find the joke funny in the first place you're stuck with it for most of the film. These continuous plot strands include Williams  for no good reason  worrying that he's changing sex, and Kenneth Cope in drag. Like the stagy physical pratt falls, the whole thing feels more contrived than in other movies, and lacking in cast interest. Continuing this theme, Matron lacks the customary pun and innuendo format, largely opting for characterisation and consequence to provide the humour. In fact, the somewhat puerile series of laboured misunderstandings and forced circumstance reminds one more of Terry and June ... so it's appropriate that Terry Scott is present, mugging futilely throughout.
Some dialogue exchanges have a bit of the old magic, such as this between Scott and Cope: "What about a little drink?" "Oh, no, no, I never touch it." "Oh. Cigarette then?" "No, I never touch them." "That leaves only one thing to offer you." "I never touch that either." That said, while a funny man in his own right (livening up the duller episodes of Randall and Hopkirk (Deceased) no end), you do feel that Cope isn't quite tapped in to the self-parodying Carry On idealology and that Bernard Bresslaw dressed as a nurse would be far funnier. This does actually happen, in part, though only for the last fifteen minutes.
Williams attempting to seduce Hattie Jacques while Charles Hawtrey is hiding in a cupboard is pure drawer room farce, but lacks the irony to carry it off. That said, Williams's description of premarital relations is priceless: "You don't just go into the shop and buy enough for the whole room, you tear yourself off a little strip and try it first!" "That may be so," counters Jacques, "but you're not going to stick me up against a wall." Williams really comes to life in his scenes with Hattie, and you can never get bored of hearing a tin whistle whenever someone accidentally flashes their knickers.
Carry On Matron is not a bad film by any means, just a crushingly bog-standard one.