This was an adaptation of a 1905 Danish detective story. One problem that is consistently noted throughout not only this series but also in other productions by British television is the portrayal of other nationalities by the English. This is certainly NOT unique to their acting system; I have seen American actors (such as Clint Eastwood and John Wayne) portray, or attempt to portray characters of other nationalities, so poorly that one starts trying to stifle one's attempt at laughing out loud. Fortunately, in this case, the cultures of Denmark and England (like their languages) are close enough that a reasonably good British actor can portray a Danish character reasonably well enough to pass muster. And, so it is in this story. Now, whether or not Catherine Schell (a Hungarian native) acted well enough to have one believe she is a native Russian aristocrat is another story.
Anyway, the basis of the print story was unusual for its time; to wit, that a conclusion that appears correct may not actually be so. I am not referring to a "false lead"; which appears quite often in detective stories (particulary the ones by A. Conan Doyle that have you-know-who appearing in them), but rather the "different interpretation" of events that is a somewhat common theme today. Example- a man whips tout a knife from his pocket and cuts another man's throat. sounds like a case of murder or attempted murder. Right? Not necessarily; when one realizes the man with the knife is a doctor and the person whose throat is cut was unable to breath to horrible swelling of his neck due to an allergic reaction to a bee sting. The cut provides a pathway for air.
This story is not as openly dramatic as an emergency tracheotomy but the stakes are as high. A great story; even with British actors acting as Danes, with a conclusion that provides food for thought.