The Crimson Flash — Roy J. Snell

Ten-year-old twins Marjory and Margaret MacDonald may be wealthy beyond most people's wildest dreams, but that doesn't mean that they don't like to have fun, just like other kids their age. While attending the circus under the watchful eye of their personal bodyguard, Johnny Thompson, the twins fall victim to a seemingly random accident. Johnny sets off on a cross-country adventure to try to get to the bottom of the mystery.