Miss Forbes, customarily a rather slipshod producer in her own name and as Tobias Wells, has come up with something special this time: an uncluttered period piece--Boston, 1918--that trolleys along on naive charm, on ingratiating detail, and on the broad shoulders of Mary Katherine Donovan. Through accident and spunk, big-boned, under-educated Mary Kate becomes ""Dear Priscilla,"" lovelorn columnist of the Boston Globe, and her green Irish eyes are smiling away with new friends and opportunities till she starts getting letters from a misogynist killer who signs himself ""Miserable Mouse."" Mouse has killed thrice--angered by his ladies' impurities--and warns Mary Kate to guard her virtue or become the next victim. With a more tremulous heroine, this plot could easily get all fluttery and Gothicky, but Mary Kate remains curt and direct, even when the psychopath (there are at least six likely prospects, including a dandy, a cop, and two newsmen) turns out to be close to home. No detection to speak of, but with Mary Kate, the Old Howard, and Sigmund Romberg's Maytime--who cares?