TONIGHT'S CBS telemovie, the fact-based "A Mother's Instinct," is a docudrama like so many others: a tale of a seemingly good marriage gone dangerously sour, with a familiar TV star in the top role.

One of the differences this time, though, is that the story involves not only acrimony and child custody, but bigamy. Another is that, because of strong acting and an unusually dramatic climax, "A Mother's Instinct" actually draws you into its story.

Veteran TV actress Lindsay Wagner is the biggest name in this movie (9 p.m.), but after appearing in the opening scene, she vanishes for almost the entire first half. Instead, the focus is on Debrah Farentino, star of "Earth 2," who plays Holly. She meets and marries a charming and quiet widower, Carl (played by John Terry), who has two young sons.

They build a home and a small business together, but Carl begins getting testy when Holly questions him about locked filing cabinets and his sometimes conflicting stories about his past life and wife.

"Marriage is based on trust," he tells her irritably, even as he's locking drawers to keep her out of them.

When she probes too deeply, one day he ups and leaves, taking their two sons with him.

Holly reacts to this abandonment by hiring a private detective, who finds not Carl, but Carl's "dead" wife the woman played by Wagner and a previous victim of Carl's vanishing act. Their initial, awkward meeting is the opening scene in "A Mother's Instinct," and after a long flashback leading to that moment, the rest of the drama details how the two wives work together to pick up Carl's trail.

In outline form, this story, though true, sounds similar to dozens of other factual or fictional telemovies. "A Mother's Instinct" breaks from the pack partly because the women's relationship is as atypical as their detective work, and because Farentino, Wagner and Terry all play their roles credibly and with restraint.

And when the paths of the three characters they play ultimately converge, the rest of "A Mother's Instinct" plays out with a high level of drama, emotion and unpredictability.

Teleplay author Norman Morrill and director Sam Pillsbury deserve much credit, too, for the project's success, making it a far better viewing experience than even the network's previews suggest.

DAVID BIANCULLI