When Alvin's teacher asks each member of the class to do something to improve Riverton's environment, Alvin and his best friend Shoie decide to tackle the town's two biggest polluters, Biggs Chemical Company and Blaha Foundry. They disguise themselves as Superweasel, a superhuman friend of man and foe of pollution. During a series of secret nighttime escapades, they and the tag-along Pest, Alvin's sister, block a drain which has been releasing chemical wastes into Riverton's Weasel River and close a foundry smokestack which has been discharging harmful soot into the air. When an outbreak of vandalism hits Riverton and Superweasel is blamed, Alvin and his Magnificent Brain are put to the test. The climax takes place during a town-wide clean-up drive as Alvin reveals Superweasel's true identity and unmasks the real vandal. Once more series fans will enjoy Hicks' established formula based on Alvin's ingenious inventions.