In book 3 of the Infinity Ring series, Sera, Riq, and Dak travel to the year 1850 in the state of Maryland.  Their mission is to fix a break in the time continuum that has something to do with the Underground Railroad and Harriett Tubman.  With Riq being of African descent, he is immediately captured and fixed to be sold at a slave auction with a black woman and her two children.  While waiting for the auction, Riq discovers that the woman is an ancestor of his and he must help her to fulfill her destiny if he is to even exist in the future.  The three time travelers work to ensure the right people get to Canada to freedom and that the Underground Railroad is successful.  McMann does a great job of keeping the action moving as well as characterization.  This is a great way for young readers to get to know more about American history.  Each book takes the three friends to different eras throughout history.