Dr. Mike and her family receive an unexpected gift from Boston, a small poodle unsuited to life on the frontier. Brian has his first crush on Sara, Preston is embarrassed by a painful kidney stone, and the Reverend plans a living nativity.

When Sully is offered a job helping to survey the new National Park at Yellowstone in Wyoming, Dr. Mike is faced with the prospect of leaving her practice and all their friends and moving into the wilderness. With Sully no longer acting as the local Indian agent and banned from the Indian reservation, Dr. Mike realizes that it is important for him to have work that is meaningful to him. Leaving everything and everyone she knows and taking their new baby into the wilderness, however, is a daunting prospect for her. Meanwhile, as Matthew and Emma think about marriage, Brian agonizes about his first date at the Sweethearts' Dance, and Dorothy teaches Cloud Dancing some ball room dances while he provides her with background on Indian customs for a book she is writing.

When Sully opposes the construction of a nearby dam, he finds himself on the other side of the law and behind bars. Sully joins forces with Preston to try and peacefully stop the dam construction which will mean the end of hundreds of acres of countryside. But when Sully hears that the men Preston's hired to hijack a shipment of dynamite are determined to kill any witnesses, Sully rides with them in order to prevent the murders. However, when witnesses attest to the fact that Sully was riding with the hijackers, Matthew is forced to arrest him and Sully must go to court where Judge Webster finds him guilty and throws him in jail, creating discord in the family.

Dr. Mike's mother and oldest sister visit for thanksgiving just as an outbreak of diphtheria hits Colorado Springs.