Before it takes an appalling turn for the vicious, "The Silent Partner" seems an uncommonly clever and gripping suspense thriller. Even after the story threatens to self-destruct, you fight the impulse to suffer a major letdown, for the sake of the swell nerve-racking time you've been having up to that point.

A Canadian tax-shelter production, "The Silent Partner" was shot on location in Toronto, using the shoppingmalls of the Eaton Centre as a principal location and the wintry night city as an evocative backdrop for a tense, eerie game of cat-and-mouse. Elliott Gould plays the mouse, a deceptively docile bank teller who indulges a hidden, astute streak of larceny. Christopher Plummer plays the cat, a ruthless bank robber who doesn't take kindly to being outsmarted.

Gould's character, Miles, works at a branch office in the Eaton Centre. He is generally ignored and underestimated by co-workers, including the one he would most like to ingratiate himself with -- a divorced bank officer, Julie, played by Susannah York. Involved in a futureless affair with the complacently married bank manager, Julie expresses the prevailing misreading of Miles when asked about him by a new girl in the office: "Let's just say the total is somewhat less than the parts."

Like many casual remarks in Curtis Hanson's crisp, witty screenplay, derived from the mystery novel "Think of a Number" by Anders Bodelson, this comment is calculated to boomerang. Miles may be diffident and solitary -- a tendecy underlined by his choice of hobbies, collecting rare fish and working chess problems -- but he is far from harmless. Moreover, he's so exceptionally observant and foresigted that he anticipates a bank robbery and indentifies the robber in advance.

Thus Miles is one step ahead with a little criminal scheme of his own by the time the robber makes his move. Miles surreptiously sets aside $50,000 for himself and pretends it has been stolen along with the amount lifted by the robber before Miles sent him scurrying out the door by handing over the bills triggering the alarm system.

Miles manages to hide the stolen money and tries to sit tight. The police seem a little suspicious of him, although they're more likely to suspect him of being a cowardly clod than a rip-off artist. Appearing on television in the wake of the robbery brings Miles a certain celebrity. It also brings him a menacing telephone call from the robber, Plummer as a psychopath named Harry, who is somewhat amused at having been conned but has no intention of letting an amateur get away with such presumption. "Think of a number," Harry quips and threatens to do very nasty things if Miles doesn't hand over the extra $50,000 that went astray.

To stress the fact that Harry is a sadistic brute and set up a necessary plot manuever, we see him beating up a young woman. The demonstration isn't necessary: Plummer's arrogant presence and insinuating diction are frightening enough.

In taking measures to outwit Harry, Miles tends to become more and more implicated in Harry's criminality. It is a familiar literary theme -- the psychological affinities between a truly dissolute, criminal mentality and a decent man drawn by folly, chance or desperation into his perilous but seductively exciting orbit.

The battle of wits is spun out with considerable ingenuity and unflagging dramatic interest. But Miles is allowed to sink deeper into the muck of Harry's evil calculations and reprisals than he should, at least in a genre entertainment designed to resolve anxiety at the fadeout. There comes a point in the story when Miles really must make a clean breast of things and seek resources greater than his own in order to neutralize Harry.

It's not merely conventional morality or storytelling expectations that require such a heroic gesture. Fundamental decency demands it. Miles owes his life to a character who proves loyal and brave enough to take the fall for him. His ultimate failureto repay this debt in some fashion, however symbolic and inadequate, leaves one with a bad taste. I wouldn't be surprised if this ethical blind spot has been responsible for the film's failure to catch on in earlier engagements around the country.

The director, Daryl Duke, made a promising feature debut several years ago with "Payday" (not a bad title for his new film) before returning to steadier work in television. "The Silent Partner" demonstrates that his skills have only improved. Duke combines a keen sense of social settings and character interplay with a jolting, shuddery aptitude for obligatory shocks, thrills and apprehensions.

Miles is so effectively "located" at the bank and his bachelor apartment that, given his surroundings, associations and frustrations, it's not difficult to see why he is tempted to act out a widespread fantasy of how crime could be made to pay.

In addition to the three costars, Duke draws expert performances from several supporting players: Celine Lomez as a sultry girl-in-the-middle and Gail Dahms, Michael Kirby, John Candy and Michael Donaghue as Miles' co-workers. Billy Williams' lighting and Oscar Peterson's score are admirable enhancements, consistently encouraging a heightened sense of fearful anticipation.

If only the filmmakers had anticipated the amoral traps in the story, "The Silent Partner" might have been a classic.