The damage from a broken water main does more than wreak havoc in Midtown Manhattan--it uncovers the grave of Milos Tryvomanic, a.k.a. Milos the Magnificent, a vaudeville crossbow- shooter who's been reposing, along with the tools of his trade, in a sub-basement of the Old Knickerbocker Theater since 1933. Now an inoffensive Vietnamese ÇmigrÇ is dead, someone has stolen Milos's crossbow and arrows, and it's open season on the habituÇs of the Old Knick--especially on megalomaniac producer Sir John Victor Holland, who's rushing to renovate the aging theater for his latest premiere, reclaiming it from its present squalor to make it a worthy neighbor for Times Square's Disney and Gap outlets. The theater's riddled with suspects who either hate Holland (the music critic whose lyrics he swiped, the disgruntled London leading lady he dumped for a wispy, collagen-enhanced Hollywood star) or who squirm in distaste at the very thought of his new production, Casablanca: The Musical. Enter Capt. Bill Donovan, NYPD, who, though no fan of Holland's, will nab the well-nigh invisible killer in time to save the theater and celebrate his nuptials under the most unexpected circumstances. Jahn (Murder at the Museum of Natural History, 1994, etc.) supplies reams of eye-opening historical detail (factual) and suitably bitchy chitchat (barely fictional) in lieu of a more arresting puzzle.