When Vincent van Gogh's Portrait of Monsieur Trabuc turns up unexpectedly at the Metropolitan Museum of Art--a $50 million painting shipped from Argentina via UPS, like an ordinary package--the case goes to Clay Ryder, the NYPD Major Case Squad detective assigned to art theft.   Ryder discovers that in Paris, late 1944, a Jewish widow accused a German SS officer of stealing the painting. The officer was reported to have died in a car crash at the war's end, and the whereabouts of the Trabuc between then and now remain a mystery. Ryder's search for the widow's heirs leads him to Rachel Meredith, who teaches at NYU. The museum presents the painting to her in a spectacular public ceremony that winds up on the front page of newspapers around the world.   Though the case is closed, Ryder can't seem to shake it. When Rachel Meredith is attacked, she calls on him; what might be a simple assault doesn't quite add up. And he still wonders who sent the van Gogh from Argentina.  One of his most reliable contacts in the art world floats a theory that ties the van Gogh portrait to a black market auction in the 70's that might have involved a Swiss art dealer and an international crime kingpin with unlimited cash. Then Israel's Mossad pays Ryder a clandestine visit; the news splash about the van Gogh is the first link they've had to the SS officer in decades.   Meanwhile, art dealers, auction houses, and museums vie to buy the van Gogh from Rachel Meredith. When she refuses to sell, the situation goes from predatory to violent. Ryder has to race against time to outmaneuver a cunning mastermind who will resort to as many murders as it takes to get hold of the Trabuc.