Israel has been decimated, the PLO has triumphed, and an Israeli refugee plots revenge--in a futuristic thriller that's gripping in its first half, foolish and sketchy in its second. Isaac Rehv, soldier and professor, is now a waiter and a live-in night watchman at a Soho art gallery in N.Y.; his wife and daughter were rape/murdered by the Arabs; he is one of thousands of Israeli refugees in the US. But Isaac is more numb than vengeful in this novel's first half--as he reluctantly agrees to help old ex-Haganah leader Harry in a plot to assassinate commando-leader Abu Fahoum: the first attempt (at the restaurant where Isaac works) fails miserably; the Palestinians set out to kill Isaac. And though Abu is ultimately finished off in a sordid porno-theater murder, Isaac himself has been non-functional. (Harry, who'll soon be dead, says: ""I have no use for you."") Then, however, while a ruthless, obsessed US agent named Krebs starts closing in on him, Isaac finally snaps into fevered action: he does a neat heist to acquire a lot of money, and he concocts a strange plan involving his impregnation of a black prostitute. So far, so good--with taut suspense, creepy mystery, and nicely realistic details (the trendy-Soho atmosphere) to keep the futuristic premise pinned down. But then, unfortunately, we jump ahead 15 years to learn what Isaac's secret plan is: he has fathered a coffee-colored son so that this child--Paul--can be raised to become the Muslim world's teenage messiah: the Mahdi! (Paul is reared in an isolated Canadian retreat, with Arabic studies as his major.) And then, you see, the phony Mahdi will lead the Arabs to holy war and self-destruction--a scheme which Abrahams (The Fury of Rachel Monette) never makes even half-believable. (A. J. Quinnell did better with a similar idea in The Mahdi, 1981.) Thus, the hurried second half here--Paul's rise to Mahdi-dom in the Sudan, Isaac's descent into madness, Krebs' continuing demon-pursuit--lapses into cartoon, with Paul's thin characterization especially glaring. (He undergoes an implausible transformation.) And though this thriller offers abundant evidence of Abrahams' gritty suspense talent, only undemanding readers will stay tuned when the strong end-of-Israel premise gives way to spotty comicbook fantasy.