Ranger in time

Ranger, a golden retriever, could have been a great search-and-rescue-dog except for the squirrels--but one day he unearths a mysterious box and finds himself transported back to the year 1850 where his faithful service is really needed by a family traveling west along the Oregon Trail.

This time the mysterious box that Ranger the Golden retriever found transports him to a Maryland plantation before the Civil War, where he must help a young house slave named Sarah and her younger brother Jesse find their way to the Underground Railroad and North to freedom, before Jesse is sold to a plantation further South.