Students are giving oral presentations about personal heroes. Ronnie's subject is his grandfather's dog.
Years earlier, a puppy is sent from Japan to the United States, but escapes when his cage falls off the baggage cart at an American train station. Professor Parker Wilson finds the abandoned dog and when the station controller refuses to take the puppy, he takes it home with the intention of returning the animal to its owner. Initially, Cate Parker does not want them to keep the puppy. Parker learns that the dog is an Akita. The dog has not been claimed when he returns to the station the following morning, so he takes him to the college, where Ken, a Japanese professor, suggests that perhaps the two are meant to be together. He translates the symbol on the collar as 'Hachi'—Japanese for the number 8—signifying good fortune. Parker decides to call the dog Hachikō. Parker attempts to play fetch with Hachi, but he refuses to join in. Cate receives a call from someone wishing to adopt the puppy, but having seen how close her husband is with Hachi, she tells the caller, "Hachi has already been spoken for. "
Parker contiues to be mystified by Hachi's refusal to do dog-like activities like chase and fetch. One morning, Parker leaves for work and Hachi follows him to the train station; he refuses to leave until Parker walks him home. Later in the afternoon, Hachi walks to the train station, to wait patiently for Parker to come home. Parker relents and walks Hachi to the station every morning. After Parker's train departs, Hachi walks home, returning in the afternoon to see his master's train arrive and go home together. They continue to do this every day.
One day Parker gets ready to leave and Hachi barks at him and refuses to join him. When Parker does leave, Hachi chases him while holding his ball. Parker is surprised but pleased that Hachi is finally willing to play fetch the ball with him. Not wishing to be late for college, Parker catches his train despite Hachi's barking. Later that day Parker is teaching his music class, still holding Hachi's ball, when he suddenly suffers a fatal heart attack and dies.
At the train station, Hachi waits patiently as the train arrives, but there is no sign of Parker. He remains, lying in the snow, for several hours, until Parker's son-in-law Michael (Ronnie Sublett) comes to collect him. The next day, Hachi returns to the station and waits, remaining all day and all night. As time passes, Cate sells the house and Hachi is sent to live with her daughter Andy (Sarah Roemer), Michael, and their baby Ronnie. However, at the first opportunity, he escapes and eventually finds his way back to his old house and then to the train station, where he sits at his usual spot, eating hot dogs given to him by Jasjeet, a local vendor. Andy arrives soon after and takes him home, but lets him out the next day to return to the station.
For the next ten years, Hachi waits for his owner. His loyalty is profiled in the local newspaper. Cate comes back to visit Parker's grave where she meets Ken, and says she can't believe ten years have gone by. Walking past the station, she is stunned to see Hachi maintaining his vigil. Overcome with grief, Cate sits and waits for the next train with him. At home, Cate tells the now ten-year-old Ronnie about Hachi. Hachi continues his daily walk to the same spot in front of the railway station, until his final day when he continues to recollect those joyful moments of his life with his master. He imagines Parker emerging from the station and the two happily greeting each other. Hachi is then shown lying on the snow, alone and still.
Back in his classroom, Ronnie, forms his conclusion why Hachi will forever be his hero. Ronnie's story has clearly moved the class, with some students holding back tears, even those who had laughed at the beginning. After school, Ronnie is met coming off the school bus by his dad and his own puppy, also named Hachi. Ronnie and Hachi walk down the same tracks where Parker and Hachi had spent so much time together.
The closing cards reveal information about the real Hachikō, who was born in Ōdate in 1923. After the death of his owner, Hidesaburo Ueno, in 1925, Hachikō returned to the Shibuya train station the next day and every day after that for the next nine years. The final card reveals that the real Hachikō died in March 1934. But actually Hachikō died in March 8, 1935, not in 1934. A photo of his statue in front of the Shibuya train station is the last image shown before the credits roll.