When his Omni, a compass-size time traveling device malfunctions, Phineas Bogg, a Voyager (a traveler in time who helps history along), materalizes in the bedroom of an 11-year-old orphan named Jeffrey Jones in 1982. When Jeffrey's dog, Ralph, attacks Bogg and grabs his special guide book, a struggle ensues and Jeffrey falls from his apartment house window.
To save the boy, Bogg jumps out the window, catches Jeffrey -- but due to the malfunctioning Omni, they "fall to earth" in another time period and are unable to return to 1982. Because Jeffrey is a history buff and Bogg is weak on historical facts, Jeffrey becomes his "guide" until such time as Bogg can repair his Omni, which had only been programmed to 1970, and return Jeffrey to his time.
The series depicts their adventures as they travel through time to help history remain on the right track.