Lance Barlow and a slave named Shadrack, that he had rescued head West to work for the Pony Express. Pursued by the Missouri militia and a Marysville slaver, the friends ride into the threat of attacks on the Pony Express by the Paiute Indians.
"Thirteen-year-old Zane Rasmussen falls unconscious from an allergic reaction and awakens on Jackson's Island in the Mississippi River, where he's discovered by Tom Sawyer, Huckleberry Finn and freed slave Jim.
After being hailed as heroes again for claiming to have caught a Bigfoot-type monster, Tom, Huck, Becky, Jim, and twenty-first-century time traveler Zane find there may be a real monster at large.