Perhaps the most evil character you'll ever see on television is the focus of a new Mystery! (8 p.m., Ch. 7) that gets under way tonight.

In "Mother Love," series host Diana Rigg portrays the completely mad, utterly evil Helena. A 50-year-old London divorcee, Helena is obsessed with hatred for her ex-husband, love for her son and the desire to punish those she considers "disloyal." I don't want to give any plot points away, but you won't believe the things Helena does.Tonight's two-hour segment is the first of three parts. And while this British production doesn't contain any nude scenes (unlike last week's "Mystery!" on Ch. 7), it's an adult production not meant to be watched by children.

Elsewhere, Cliff and Clair are the subject of Rudy's video assignment on Cosby (7 p.m., Ch. 2); the good Father and Sister Steve meet up with a crime-solving TV priest and his nun sidekick on Father Dowling Mysteries (7 p.m., Ch. 4); The Simpsons (7 p.m., Ch. 13) celebrate Halloween in a "Tales from the Crypt" parody; Saxman Clarence Clemons appears as a high-tech thief on The Flash (7:30 p.m., Ch. 5); a surprise witness appears in Roz's case against the law firm on L.A. Law (9 p.m., Ch. 2); and Anne is blackmailed on Knots Landing (9 p.m., Ch. 5).

- LOOKING TOWARD FRIDAY: If, like me, you missed Scholastic Sports America on Thursday morning, tune in Friday at 4 p.m. (ESPN) and catch a segment on the Mountain View High cross country team; there's a Halloween episode of Night Court (8 p.m., Ch. 2); Over My Dead Body (8 p.m., Ch. 5) premieres; and one cast member departs and another arrives on Midnight Caller (9 p.m., Ch. 2).