Appel's time-traveling hero Alex Balfour (Time After Time, 1985; Twice Upon a Time, 1988) is back--and forth--between his present-day attempts to hold down a job as a history teacher at NYU and his unwilling service in the darkest days of WW II. While Alex's live-in mate Molly Glenn researches a New York Times report on long-secret Japanese experiments (freezing, bombing, nerve gas) on American POWs, Alex--pulled first back to Pearl Harbor, then to 1943 Manhattan--comes up against the same secrets. Traveling to Princeton to find Maxwell Surrey, his old friend from the Russian Revolution, Alex finds himself face to face with Albert Einstein, who charges him with persuading President Roosevelt never to nuke civilian sites in Japan. FDR responds by sending him on a fact-finding mission to the Pacific theater--where the celebrity parade continues with Orson Welles, Betty Grable (""I've got more tricks than a Chinese whore""), and Lt. Jack Kennedy, whose life Alex saves as Pt- 109 is run down by a Jap destroyer--before two captured Japanese prisoners, claiming that Japan has its own nuclear program, offer to take Alex back to Japan under their protection so that he can confirm its existence to Roosevelt and press for the American bombing, saving thousands of casualties on both sides. Naturally, there are more revelations to come, some of them withheld until Alex catches up with Molly in a fortified bunker beneath contemporary Hiroshima. Though Alex's reluctant heroics can grow tiresome--how did the Allies really win the war without him?--Appel's generous dollops of history are as painlessly informative, and the tale he spins as rousing, as ever.