The final sequel to flawed, brilliant Gateway (1977) and the disappointing Beyond the Blue Event Horizon (1980)--finishing up a sprawling, cluttered, inconclusive, and curiously uncompelling trilogy. At the close of this last installment, Pohl's narrator--benevolent tycoon Robinette Broadhead--will die, transferring his mentality to a ""datafan,"" a pseudoliving state whence he can access any stored data (thus his omniscient point of view). Before he dies, though, Broadhead is still busy gathering information on the allen Heechee (who turn out to be neither very alien nor particularly interesting), aided by his boring wife and her sophisticated computer program--an unfunny and unconvincing simulation of Albert Einstein. The Heechee, whose task is to nurture and encourage intelligent life (cf. Clarke's 2001 and 2010), fear that the disturbances created by space-traveling humanity will call forth the dreaded Assassins. Who? Well, they're pure-energy beings who've caused the universe to stop expanding and start to contract, hoping that the subsequent Big Bang will improve their lifestyle. And, like the Heechee, they're hiding inside a black hole and making periodic trips outside. . .where their intent is to destroy all intelligence. As before, Pohl's odyssey is sometimes impressive, bulging with fascinating ideas. But there are too many of these ideas to fit together usefully--so, with unengaging characters and wobbly motivations, the meandering narrative has little overall purpose, drama, or tension. Not top-drawer Pohl, then, though inventive enough to reward his sizable following.