April 19, 2002 | Most mainstream movies, even most hugely entertaining ones, are like wallpaper: They don't require much effort to fully appreciate, because what you see is what you get. Since there isn't much there in the way of complexity or ambiguity -- indeed, there's next to nothing going on beyond the obvious spectacle -- you can remain a passive observer, engaged but rarely immersed.

But a movie as crafty and well-crafted as Enigma doesn't let you off so easily. You want to make sense of what's happening here? Then you have to pay close attention, connect a few A-to-B plot points without printed instructions and, most important, come to the table with at least a vague knowledge of who was aligned with whom during World War II.

This is a thinking person's entertainment - yes, I know that sounds elitist, but it's true - which means it is entertainment for any person who doesn't mind doing a little cerebral heavy lifting. Despite a few flurries of narrative fuzziness, Enigma ultimately impresses as an intelligent, involving and intricately plotted thriller about code-breaking, lovemaking and other forms of risk-taking at a top-secret cryptography center in WWII-era England.

Dougray Scott (Ever After, Mission: Impossible II) is a shade too mannered in his performance, but he nonetheless makes a compelling impression as Tom Jericho, the unlikely hero of the piece. Through flashbacks and expository dialogue, we're informed that Jericho, a brilliant but emotionally vulnerable mathematician, was recruited for work at Bletchley Park, a real-life British Intelligence headquarters during the 1940s. First among equals on a team of similarly eccentric geniuses, Jericho established himself as a most valuable player by cracking the code used in German military communications. (Transmissions are sent and received with Enigma cipher machines, devices that resemble hybrids of manual typewriters and telephone switchboards.) Unfortunately, the stress of his work -- and his rejection by Claire Romilly (Saffron Burrows), his strikingly beautiful co-worker and ex-lover -- drove Jericho to a nervous breakdown.

The bulk of Enigma plays out in March 1943, as a not-entirely-recovered Jericho is recalled to Bletchley Park. The Germans have changed their Enigma transmission code at a singularly inconvenient time: Three massive Allied shipping convoys have just left New York, loaded with supplies to sustain the British war effort. So, naturally, Jericho is expected to work more miracles of deciphering skill.

Trouble is, our hero is distracted by (a) the recent, inexplicable disappearance of Claire, (b) his discovery of undeciphered Enigma transmissions beneath the floorboards of Claire's home, and (c) the hectoring presence of Wigram (Jeremy Northam), an aggressively suave British Intelligence agent who's on the trail of a suspected mole at Bletchley.

Hoping to clear Claire's name -- or, failing that, to at least see his beloved once more -- Jericho joins forces with Hester Wallace (Kate Winslet), Claire's mousy but feisty housemate, to further his investigation. A good thing, too, because Jericho needs all the help he can get while dividing his time between amateur detective work and race-against-time deciphering.

Screenwriter Tom Stoppard (Shakespeare in Love) does a bang-up job of rethinking his source material, a well-received novel by Robert Harris. Rather than attempting to dumb down Harris' twisty plot, Stoppard artfully compresses and reconfigures, most effectively in a sequence that intercuts between code-breaking activity at Bletchley Park and Hester's solo attempt to decode Enigma transmissions. Here and elsewhere, British filmmaker Michael Apted directs with the same intelligence and cunning he brought to Gorky Park and Extreme Measures, two films that, come to think of it, could also be described as thinking persons' thrillers.

Enigma is a frankly old-fashioned movie, complete with a traditional suspense-thriller musical score (by John Barry) that deftly balances lush romanticism and dark portent. The only weak link is Saffron Burrows' unaccountably colorless performance as the supposedly irresistible Claire. She doesn't do anything wrong, strictly speaking, but she doesn't do enough that's right for the character. Fortunately, her screen time is kept to a minimum.

At its frequent best, Enigma slyly evokes the spirit of classic Alfred Hitchcock thrillers. As an aptly enigmatic British Intelligence agent, Jeremy Northam often appears to be channeling the Cary Grant of Notorious and Suspicion. And Kate Winslet's winning portrayal of the plucky heroine merits flattering comparisons to resourceful female leads in The 39 Steps and The Lady Vanishes.

It should be noted, by the way, that Winslet -- bespectacled and unabashedly zaftig in 1940s fashion -- looks appropriately unglamorous. Tabloid gossips and snippy critics will doubtless make their usual snide remarks about her weight. Those people should be beaten with sticks.