Due South

When Due South premiered last April as a two-hour TV movie, it was a one-joke affair: Straitlaced Canadian Mountie (Paul Gross) goes to Chicago and teams up with flashy local cop (David Marciano). Then this fall, CBS turned the one joke into a series, and it's done surprisingly well in the ratings. Why? Because Gross' Constable Benton Fraser has become one of the most likable characters on television. As Ray Vecchio, costar Marciano is a good actor who has steadily been reduced to playing a supporting role. That's because his character — a wisecracking tough guy — is a cliché, while Gross' Benton is more of a rarity these days: a hero who's neither cynical nor sarcastic. Sure, Benton crashes through windows and leaps across rooftops to snag criminals, but he also calls suspects ''sir'' and ''ma'am,'' and proves that you can fight crime without breaking the rules. Due South isn't great, but it's nice. B