LASSITER IS A SLOW-MOTION Magnum P.I., in tweeds. This molasses-paced fashion adventure, about an American jewel thief in 1930's London tows actor Tom Selleck through an argyle-and-herringbone wardrobe, and climaxes in a turtleneck sequence featuring a diamond heist.

Director Roger Young stretches a thin plot which involves Scotland Yard, the F.B.I., and (of course) Nazis, by punctuating it with a hackneyed score and stagey walk-ons: Clusters of bowler-hatted extras mill around aimlessly, while "protesters" wield suspiciously well-stenciled placards. In addition to its amateurism, the film is cheapened by gratuitous flashes of boob and rump' by its main performers (Selleck, Jane Seymour, and Lauren Hutton).

The script jabs at the most tolerant viewer's sensibilities with lines like "Someone else deals the cards, I just play 'em," and "If I'm not out in twenty minutes, I'm not comin' out."

Even the stylish patina of Lassiter's overall mediocrity is a Tootsie-tied scene during which Selleck pads around in a woman's feathered houserobe. It is the film's most entertaining moment.

LASSITER IS SUPPOSED to be a vehicle for Magnum P.I.'s star, but its inane script and direction render him trivial (Young, by the way, directed the 1980 pilot episode of Magnum P.I.). Selleck has failed at his second attempt to capture the larger screen (High Road to China came first). His dimpled affability has garnered an affectionate viewership, and enabled him to walk through weekly episodes of a standard-issue television series: geniality, however, is of negligible value during cinematic scenes which grind so slowly, one can hear crew members in the background phoning home for late dinners.

Some of the film's 100 minutes glide by, thanks to Seymour. An actress of her caliber could probably wrap up even Lassiter-quality dialogue during lunch breaks.