Ringworld (1970), the most celebrated work in Niven's ""Known Space"" sequence, posited a vast body of matter--enough for an entire solar system--spinning around a sun in the form of a single giant artifact of unknown origin: a continuous million-mile-wide ribbon provided with oceans, atmosphere, and vast flat projections (life-size ""maps"") of Earth and other inhabited planets. The present book takes up the puzzle some 20 years after Louis Wu's escape from the Ringworld. Kidnapped by the mate of Nessus, their two-headed alien companion of the previous voyage, Louis and his catlike ally Chmeee are transported to the Ringworld--now spinning dangerously off-center--in an attempt to discover the cause of the aberrant rotation before the world grazes its sun. Searching for clues to the design of the structure's long-vanished original engineers, they encounter various hominid and other races before finding the barely feasible, wholly appalling solution hidden beneath the ""Map of Mars."" Niven, a longstanding favorite with ""hard"" sf buffs, commands an impressive vein of invention, but his plotting here is limp and threadbare; the idea was more striking the first time around.