Initially excited when his parents break the news that the family is moving to Australia, Timmy changes his mind when he finds out Lassie would be quarantined for six months and cannot come with them.
Lassie has settled in living with Cully-and mostly occupied keeping Cully's mischievous Yorkshire terrier Silky in line-when the elderly man suffers another heart attack. Lassie fetches Doc Weaver, but in the process of his getting Cully to the hospital, Silky escapes, fair game for tourists at a tourist camp.
While Lassie and Silky wander Blue Canyon, where Silky is in danger from a marauding hawk, an old friend of the collie's shows up, Corey Stuart, who immediately heads out to find his missing friend.
Lassie finds a home with Corey Stuart of the Forest Service. After a mother Golden Eagle collides with a surveying plane Lassie helps out to keep her eaglets safe. She covers them and wards off a hungry bobcat.
At a logging camp Corey and Lassie assist with conservation efforts and some personal relationships. The foreman's young son is trying to grow up too fast. When the boy tries to operate heavy equipment, he injures himself and Lassie..
When Maude Lester's stream dries up, Lassie discovers that a family of beavers have built a dam and re-routed her stream. Lassie and Maude help the beavers move their dam to a new location to accommodate everyone's needs.
While Corey is running a bulldozer at the beach dunes Lassie meets a little girl Gail. She lives with her grandmother and with no children to play with she befriends the collie. A dangerous sandstorm puts them in peril.
Lassie, still at Corey's ranger station, befriends a herding dog on the neighboring ranch. There is concern about a pack of wild dogs and Lassie's new pal is accidentally shot while protecting a herd of sheep from the marauding animals.
Lassie battles the wild dogs for her injured friend, hides him in a cave, obtains food for him by springing a trap, and leads the hunters away from him before accompanying him back to the ranger station where he finds a new home.
Corey is checking to see if ranchers are abiding to limit grazing on forestry land. But Roland Heinz is planning to evade the plan over son Chuck's objections. An attempt to rescue a calf in quicksand puts Chuck in danger unless Lassie can help.
Corey and Lassie visit the McKinnon ranch where there's conflict. Jim's parents want him to go to college, he wants to get a job and marry. Danger lurks as a cougar stalks the cattle, injuring Ben so Jim heads out with a rifle.
Corey is sickened while surveying in Echo Canyon, and closes the forest 48 hours before the opening of tourist season. Facing the ire of the mayor and other local businessmen of the area, he searches for the source of the problem, which he is sure is in the water supply. He's right-and Lassie has ingested some of the poison.
When a bear raids food from campers at the Twin Oaks campground, Corey helps the local game warden relocate the animal to an area away from tourists, not knowing the bear left a cub behind-luckily Lassie has found the little fellow and is leading him to safety, fighting off a bobcat and hawk to do so.
When trees cut from the forest for a Christmas charity project are stolen from a church lot, Stuart plays on the suspected thieves consciences by introducing them to the charity givers and recipients .
Corey and Lassie visit a stable run by Bruce Henderson in the park. When a guest Jetta has a bad encounter with a stallion Henderson decides he must be shot but the collie has other ideas and helps the horse escape.
A boy hides his cat and kittens in a cave in the woods because his mother doesn't want them at home. Lassie plays nursemaid to all of them during a bad storm.
Lassie accompanies Ranger Hank's sister to devil's canyon for hiking and rock climbing then must protect the woman from a cougar after she falls and injures her back.
Lassie and Corey visit Casey Howell who takes care of animals no longer able to survive in the wild. A tame mynah bird flies away and the collie feels responsible. Lassie worries about a hawk and heads into a rocky canyon to find the bird.
Lassie and Corey work on a soil erosion project to restore a watershed destroyed by fire. Corey brings a collie pup for a young girl's birthday .Then the storm arrives and sweeps the pup away giving Lassie a chance to save her..
While reliving some childhood memories of trips with his father, Corey is attacked by an escaped convict. Lassie and Corey follow the convict into an old mine which collapses and they all must find a way out.
Boone Sawyer has a unique ability to communicate with nature while protecting the animals. When hunters swear out an assault complaint against Boone, Lassie visits with him while Corey searches for a solution to his problem.
Corey, Dave, and Lassie are flying over a desolate canyon when their plane is struck by lightning. Forced to parachute Corey is injured and the collie is dumped into a raging river. Separated from the rescue area Lassie is left behind.
While Corey recuperates in the hospital, Lassie journeys homeward. A prospector confines her in his barn with his mule who helps her escape. Next the collie hops a freight train and joins two runaway boys, one of whom becomes quite ill. .
Lassie continues her journey home. She fights a wolf, stops a cattle stampede, saves a man, and injures her leg. With help from a hawk Lassie finds her way home to Corey.
When Lassie finds a migrating goose that has fallen from the sky, she brings Corey to help the goose, whose devoted mate has also stayed. Lassie and the rangers race against time to get the bird an injection to fight metallic poisoning.
A carelessly tossed cigarette causes a forest fire near a dog shelter. The dogs' caretaker tries to help but he is overcome by smoke. Lassie and Corey help in his rescue.
Lassie's friend Binks accidentally gets trapped in a refrigeration truck on a hot day when the driver leaves the door ajar. Lassie follows the truck back to its home base in order to get Binks released and returned home.
Lassie finds a mischievous chimpanzee named Debbie and takes her to the ranger station where she ruins the newly-cleaned station and creates havoc outside. Lassie gets some help from a raven before the big inspection.
Dusty, a lead pack mule, is being retired but he doesn't want retirement. Instead, he breaks away and follows his pack team with Lassie close behind him. His persistence convinces his human to let him continue packing.
Nancy is a physical therapist who uses tree planting to help her patients, four children who are disabled. Corey and Lassie arrive to assist her with the kids who respond eagerly. Jimmy is the exception who is depressed about his limitations.
Ranger Corey and Lassie take a troop of boy scouts for a camping trip during which Lassie finds a young boy following them. Andre, a young Frenchman, is following the scouts to learn from them as he knows nothing about woodcraft.
