Third in a series of mysteries solved by a multiethnic trio of sixth-grade detectives in Oakland, California. Master sleuth Susannah, her friend Lucy (the ""Dr. Watson"" character, also the narrator), and their partner Knievel investigate a series of mysterious fires in a neighborhood where properties are being bought up by a development company. Multiple suspects, lots of clues (including the purple Mongoose of the title, which turns out to be a kind of bicycle), and real suspense at the end, when the kids are trapped on the roof of a burning house, make for a satisfying, if somewhat formulaic, page-turner. The one false note is the villain, who seems too sympathetic a character to be capable of the potentially deadly crime of arson. The jacket painting shows Susannah as African-American; other illustrations not seen.