This is a pretty fundamental question in TV-series production -- how do you keep a long-running show fresh without changing the dynamic that made it successful in the first place? It's an even trickier problem in "Doc Martin's" case, though, because no one can quite figure out why it's successful at all.

The show has been a hit in its native Britain since 2004, and has more recently become a global sensation, with new versions being produced in Greece, Spain and even Iran. North America has opted not to remake it but simply rerun the original version on public television stations across the continent.

There has been much discussion of why it's struck such a chord. The doctor himself is cranky and unpleasant, the setting (a small fishing village) is far from glamorous, and the star himself (Martin Clunes) is not exactly leading-man material -- an article in Britain's "Daily Mirror" discussing the show's inexplicable success described Clunes' ears as "spectacular."

Yet this formula has produced a hit, and so the producers (including Clunes' wife, Philippa Braithwaite) are reluctant to change much, including, as you point out, the main storylines.

Clunes, too, acknowledges a fear that the show could become repetitive. The show will begin shooting its seventh season soon, but Clunes told the "New York Times" that it is likely to be the last one.

"As much as I'd love to keep on doing it, I just don't know if we can. It might become a soap opera."

Clunes also told the "New York Times" that even he can't explain the show's success, except to say that the British "like our heroes a bit anti."

To call Doc Martin an "anti-hero" is fairly apt -- once a top-flight surgeon in London, he develops a fear of blood and so decides to take up a family doctor practice in a rural village, whose inhabitants he openly dislikes.

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