For 35 years Hugo and Nebula award winner Anderson ( The Boat of a Million Years ) has written stories about the Time Patrol, whose members are charged with keeping history on track, unaltered, accidentally or deliberately, by time travelers. With this first novel set in that universe, Anderson vividly demonstrates his mastery of ancient cultures and history. After several grueling cases, agent Manse Everard is taking leave of the Pleistocene Age when it is discovered that history beyond the mid-12th century has changed drastically. Manse moves uptime to the new 1980s, an era in which the Renaissance has never happened, in which the Church, via the Inquisition, has stifled most dissent and scientific development, and the New World has barely been colonized. Everard and his new love and protegee, Wanda Tamberly, determine that the key to the change lies in the premature death in battle of Roger II, count of Sicily, grandfather of Frederick II, who has thus never been born. Although the Time Patrol saves the count, they are not able to redirect history to its conventional course. Ensuring the birth of another key figure, Pope Gregory IX, becomes critical. Anderson has produced a lively, thought-provoking tale.