On furlough from his successful series of military thrillers, Poyer (The Passage, 1994, etc.) offers an overlong flight of historical fancy that pits a callow JFK against Nazi killers bent on assassinating FDR during the late days of WW II. It's the spring of 1945, and John Kennedy is convalescing stateside after the PT boat he skippered was run down and sunk off Guadalcanal. While unimpressed by the heroic public image that Joe Kennedy has created for his eldest surviving son, JFK's superiors (alerted to the possibility of an enemy agent in the White House) put him on the ailing FDR's personal staff as their eyes and ears. The feckless Navy lieutenant clashes constantly with Secret Servicemen, top aides, the press corps, and others, but he delights the gregarious, larger-than-life chief executive. In the meantime, a U-boat has landed a pair of operatives on the Virginia coast in furtherance of a Third Reich plot to murder the president, place the blame on Stalin, and avert Armageddon by joining forces with the US against the Soviet Union. One member of this two-man band, a disaffected SS colonel, goes straight to the FBI, but J. Edgar Hoover (in the conviction his country has a postwar rendezvous with Communism) doesn't act on the information. The other, an American- born Russian who betrays his NKVD masters out of hatred for the USSR's red regime, perseveres and very nearly does for FDR on several occasions; his undercover accomplice fails as well. On April 12, 1945, however, their efforts pay off, and the President dies, not of a stroke, but in a hail of bullets while posing for a portrait painter in Warm Springs. Young Kennedy dispatches the gunman, while a dea ex machina deals summarily with his co- conspirator and the 50-year coverup begins.... Despite a few tension-filled moments, an amalgam of fact and fantasy more lengthy than stirring.