In San Diego in 1918, as deadly influenza and World War I take their toll, sixteen-year-old Mary Shelley Black watches desperate mourners flock to séances and spirit photographers for comfort and, despite her scientific leanings, must consider if ghosts are real when her first love, killed in battle, returns.

Is this town haunted? That's what Jack and Annie wonder when the magic tree house whisks them to the Wild West. Soon they run into horse thieves, rattlesnakes, a cowboy named Slim ... and maybe even a ghost! Can they solve the riddle that lets them get along home?