Even your average TV suspenser--not to mention the gamut of pro writers from Michael Innes to Donald Westlake--generates a surer aura of authenticity around an art-forgery caper than does this daub of amateurish cuteness by an ""expert on art forgery."" Gallery-owner Gillian Dwyer, too pure to market faddish modern stuff (ignorant, dated jibes at modern art abound here), knocks off slick imitation mini-Chagalls till adventurer Andrew Macrae happens along to make her cry out ""in the night. . . in a high, wild way."" Sexy Andrew convinces Gillian that she can handle something bigger--like a Jean-Pierre Biscay (1902-1938), which would be worth close to a million. Gillian does indeed manage it--old canvas, old frame, baked for age, etc.--but the fake is stolen, so she does another Biscay, which, authenticated by a foxy middleman with a ridiculous ease, is sold to a rich collector. Complications set in, of course, not the least of which are Gillian's moralistic twitchings and the arrival of the real Biscay, who didn't die in 1938 after all (remember Monty Woolley in Holy Matrimony?). If, as so obviously intended, Andrew and Gillian (favorite expression: ""Horsepiss"") were youthfully irresistible, this might canter along despite the dumbness. But they're not, this doesn't, and Gillian's fakeries are the least phony items on the premises.