Lassie was born and raised in a pet store. He liked it there and every time he was adopted, he ran back to it. In the end, no one wanted him because he was no longer a cute, little puppy.
He meets the young woman, Akutsu in the store and becomes very fond of her. Akutsu takes him for walks, plays with him and feeds him. However, Lassie is sold to another family before Akutsu could afford to buy him. Now Lassie begins his journey back to find Akutsu. On his way, he meets the German Shepherd, German. First, he saves him, but later Lassie returns the favor. He helps German learn to trust humans again and helps him find a new home with the boy, Shingo Yaguchi.
Afterwards, he continues his journey and meets a Shiba mix called Chiyomaru. He too helps him get back and reunited with his owner. After this long journey, he's taken care of by Chiyomaru's owner, Seinosuke Andō. Seinosuke calls the pet store to find out if they are missing a Collie. It turns out that it's the same store Lassie originally came from and where Akutsu works. He's then finally reunited with Akutsu to live happily ever after.