Newman builds on the best elements of her interconnected Planetfall series for this superb fourth volume (after Before Mars), which centers on a covert investigation into nuclear genocide. Emotionally cold Dee seethes, wondering who aboard the spaceship Atlas 2 ordered the secret nuclear launch that ravaged Earth. The only people who observed the detonation are Dee; her best friend, Carl; and Carl-s -sort-of lover,- Travis. Six months into their 20-year journey to a new world, an unexpected offer of a job analyzing entertainment usage allows her access to the ship-s computer systems, which lets her seek answers. She is also invited to elite games where players rely on their real-world physical abilities in virtual space, and encounters a cagey virtual guide. After her first foray, she is disturbed that a man she killed in the game died in the real world. Carl, a former detective, investigates the man-s death, but the mysterious guide uses impressive data hacking skills to stymie him and protect Dee, while inviting Dee to take revenge on the game-s designers for setting her up to be a killer. Despite her fear of being caught, Dee agrees to aid the guide-s scheme. Tense moments mount as she learns more about her incognito ally and the ship-s leadership, leading to a chilling, ethically complex ending. This winning combination of intriguing mystery and inventive SF richly rewards Newman-s longtime readers and will lure others to the series. Agent: Jennifer Udden, Barry Goldblatt Literary. (Apr.)