It's 1860. Annie Dawson loves living on the Red Buttes Pony Express Station, with daring riders like her friend Billy Cody racing through to carry the mail coast to coast. But more than anything, Annie loves the pony Magpie. When Magpie suddenly turns wild and dangerous, Pa says she has to be shot. Annie's sure someone is poisoning the pony -- but how can she prove it? And can she prove it in time to save Magpie?