Zoe and Harvey are amazed to discover a rare baby white buffalo in the park. The animal's presence has to be kept secret to protect it, but when they realize they have accidentally put two poachers on the buffalo's trail, Zoe and Harvey set out to make amends by preventing the scoundrels from capturing it. And to do this, they can always count on their closest ally: Lassie!

When a mysterious epidemic breaks out in the park, Zoe is determined to find out what has caused it so that Lassie and other animals don't fall ill. Zoe soon loses patience with Harvey's scientific approach and his insistence on seeking out proof.

Impatient to see her friend Judith again, who has left the valley several years hence, Zoe reminisces about their intrepid adventures in the Park. But their reunion takes an unexpected turn for Zoe when she discovers her friend is now confined to a wheelchair.

When Mrs. Lee opens a letter that Zoe, Harvey and Lassie bring her, she learns that she is the last living heir to a former mining concession. When Mrs. Lee lets them read her journal from the period, the kids and Lassie set out in search of the abandoned mine to discover the "magnificent treasure" that the journal hints is hidden there.

When Lassie spectacularly rescues a herd of escaped zebras, a circus director is so impressed by her performance that he insists that Lassie join his troupe. Naturally, he comes up against Zoe's refusal: Lassie is not a circus animal; she is a member of the family. When her beloved Lassie disappears, Zoe is convinced that the circus director is behind the disappearance.

To perpetuate his father's own passion, Harvey decides to enter the photo competition for the Park's Visitor's Guide. When he finds out about his rival - Samantha's- high-tech equipment, Harvey asks Doctor Dudley to tweak his camera to be able to take spectacular photos, even if it means putting himself in danger.

Zoe, Harvey, Graham, Beth and Lassie are forced to interrupt their hike on the valley glacier when they notice that its peak is in danger of collapsing. Beth seems hugely upset at the idea. Later, when Lassie notices Beth's car parked at the foot of the glacier, Harvey realizes that his mother has been hiding something from him. Despite the danger, he sets out to look for her with Zoe and Lassie.

When Zoe and Harvey visit the new dam at the edge of the park, the noises emanating from the construction upset Lassie who refuses to go any closer. When she understands the valley can be flooded due to Nick's error, Lassie has to draw on her inner strength to overcome the stress the sounds of the dam are causing her and save the kids trapped inside the dam.

While helping establish a new nature preserve on the edge of the Park, Zoe, Harvey and Lassie meet the scientist, Doctor Dudley, an eccentric recluse who has recently moved to the valley. Convinced the whole scheme is a fraud, the doctor is reticent about the project and refuses to sell his land to the environmental organization behind the nature preserve.

Zoe decides to pursue the family tradition by winning the Park's legendary blueberry pie competition. To achieve this she has to bring a special personal touch to the family's award-winning recipe. But when Samantha arrives with the recipe by a top-chef in hand, Zoe begins to doubt she has the ability to find her own special twist to make her pie the best.

Zoe and Harvey set out to investigate theft from the park archaeological site. The pair soon finds themselves in big danger and Lassie is faced with a huge dilemma when she finds she cannot save both of them. Lassie has to use great cunning and aplomb to save the kids from their predicament while also putting a stop to the thefts.

While Harvey is on vacation in the Caribbean for the winter, Zoe has to help her father to secure the park before the arrival of a blizzard. When Zoe leaves the rangers' station to save a wolf-cub, she becomes trapped in the snow storm. Fortunately, Harvey is never far away and Lassie, as ever, is there to help.

Today is Halloween! Zoe seizes on the opportunity to prove to her mother how grown-up and responsible she is by chaperoning Mrs. Lee's niece on her trick-or-treat trip to the neighboring town alongside Harvey and Lassie. Zoe and Harvey are soon overwhelmed by the little girl's unpredictable behavior and soon lose track of her! Fortunately, Lassie is on hand to save the day.

After Lassie helps a young horse-rider to control her unruly mustang, Zoe and Sarah take her in as the horse rests. But when two brothers turn up in the valley in search of their stolen mustang ,Zoe realizes it's her new-found friend, who has now suddenly disappeared. Lassie encourages Zoe to listen to her heart and our heroic trio sets off in search of the girl.

It's Harvey's birthday and his mother offers him his maiden flight in a hot-air balloon. Of course, Harvey instantly thinks to invite Zoe along to share his dream-come-true! While Harvey is over the moon, Zoe is afraid and she shares her fear of flying with Lassie. She cannot however bring herself to confess her fear to Harvey and runs away just before take-off.

Now that Zoe and Harvey are old enough to take part in the Grand Mountain treasure hunt to celebrate the founding of the park, they are determined to win the race with Lassie's help. While Zoe shows her headstrong nature by turning down her mother's help, doubt soon sets in when her big rival, Samantha, somehow manages to stay one step ahead of our heroes on each challenge.

Olive and her father listen as Zoe relates how she and Lassie were trapped by a forest fire after Harvey had left them to seek help. While young Lassie demonstrated her extraordinary devotion to help the young Zoe from her predicament, our heroine once more demonstrates her mettle when Olive's puppy gets lost in the park and Zoe is called upon to find it.

It’s "Adopt-a-Puppy" day and Zoe and Harvey host Olive and her father who have come in search of a new family friend. But the puppy Olive falls for does not turn out to be as courageous and Lassie-like as her father would have liked. Zoe relives the first time she met her beloved dog, at the same time as her best friend Harvey, who had just arrived in the valley!