The notion of a dumb person in Portwenn certainly isn't unusual...in fact, it's a given. But in this episode, two complete prats arrive and throw everything into a tizzy.
The first visitor is a doctor who has arrived to test Doc Martin's diagnostic skills....which seems like a waste of time since this is the doctor's biggest strength. In fact, the visiting doctor didn't seem nearly as skilled and perhaps a bit dim. The second is Mark Mylow, who was a police constable who was a bit less annoying and dim than Penhale...though this isn't exactly setting the bar very high, is it?! Together, they use their limited skills and even more limited intelligence to investigate a case involving stolen farm equipment. Hopefully the pair will remember to breathe.
Seeing Mylow again was a shock in this episode because he was a regular on the show long ago....and I never knew why he left the show. And, IMDB doesn't seem to indicate either why he'd leave a successful program. But this time, the writer dumbed down his character more than he used to be...to the point that he was practically a talking turnip. If you like seeing two Penhales, then you'll be thrilled. I just wanted the two to wander off somewhere never to return, as these two characters seem awfully broadly written.
Overall, a decent episode whose quality depends on how much you like watching dumb policeman. I found them a big annoying, hence my slightly lower than average (for this series) score of 7.