After a pair of expensive historical epics (Gangs of New York, The Aviator) Martin Scorsese returns to the well for this blistering crime thriller about cops and robbers in South Boston. A remake of the Hong Kong hit Infernal Affairs (2002), it stars Jack Nicholson as a ruthless mobster, Martin Sheen as captain of an undercover police unit, and Matt Damon and Leonardo DiCaprio as the young men they send to spy on each other. Neither spy knows the other's identity, and their cover is so deep each runs the risk of being stranded in his new life. It's a classic doppelganger setup, reminiscent of Hitchcock's Strangers on a Train, though it may be the least moral story Scorsese has ever taken on, functioning simultaneously as a thrill ride and a coldly cerebral Skinnerian exercise. With Mark Wahlberg, Ray Winstone, Vera Farmiga, and Alec Baldwin. R, 149 min. Reviewed this week in Section 1. a Century 12 and CineArts 6, Chatham 14, Crown Village 18, Davis, Ford City, Gardens 7-13, Lake, Lawndale, Lincoln Village, Norridge, Pickwick, River East 21, 62nd & Western, Village North, Webster Place.