In the modern day, a class full of young students are giving oral presentations about personal heroes. A boy named Ronnie (Kevin DeCoste) stands up and begins to tell of 'Hachiko', his grandfather's dog. Years before, an Akita puppy is sent from Japan to the United States, but his cage falls off the baggage cart at an American train station, where he is found by college professor Parker Wilson (Richard Gere). Parker is instantly captivated by the dog. When Carl (Jason Alexander), the station controller, refuses to take him, Parker takes the puppy home overnight. His wife Cate (Joan Allen) is insistent about not keeping the puppy.

The next day Parker expects that someone will have contacted the train station, but no one has. He sneaks the pup onto the train and takes him to work, where a Japanese college professor, Ken (Cary-Hiroyuki Tagawa), translates the symbol on the pup's collar as 'Hachi', Japanese for 'good fortune', and the number 8. Parker decides to call the dog 'Hachi'. Ken points out that perhaps the two are meant to be together. Parker attempts to play fetch with Hachi, but he refuses to join in. Meanwhile Cate receives a call about someone wanting to adopt Hachi. After seeing how close her husband has come to Hachi, however, Cate tells the caller that Hachi has already been adopted.

A few years later, Hachi and Parker are as close as ever. Parker, however, is still mystified by Hachi's refusal to do normal, dog-like things like chase and retrieve a ball. Ken advises him that Hachi will only bring him the ball for a special reason. One morning, Parker leaves for work and Hachi sneaks out and follows him to the train station, where he refuses to leave until Parker walks him home. That afternoon, Hachi sneaks out again and walks to the train station, waiting patiently for Parker's train to come in. Eventually Parker relents and walks Hachi to the station every morning, where he leaves on the train. Hachi leaves after Parker's safe departure, but comes back in the afternoon to see his master's train arrive and walk with him home again. This continues for some time, until one afternoon Parker attempts to leave, but Hachi barks and refuses to go with him. Parker eventually leaves without him, but Hachi chases him, holding his ball. Parker is surprised but pleased that Hachi is finally willing to play fetch the ball with him. Worried that he will be late for the college, Professor Parker leaves on the train despite Hachi barking at him. At work that day Parker, still holding Hachi's ball, is teaching his music class when he suddenly suffers a fatal heart attack.

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 new 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 Jas (Erick Avari), 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 nine years, Hachi waits for his owner. His loyalty is profiled in the local newspaper. Years after Parker's death, Cate comes back to visit Parker's grave when she catches sight of Hachi, now old and achy, waiting at the station. She gets emotional and sits next to Hachi until the next train comes. Hachi returns to the train station late at night and closes his eyes for the last time. Then, Parker walks out of the station and greets him as if nothing has changed at all, and the two reunite as their spirits rise up to Heaven to be together forever.

The film then shows Ronnie, back in his classroom, making his conclusion of why Hachi will forever be his hero. He then meets up with his own Akita puppy, named Hachi, to walk down the same tracks where Parker and Hachi spent so many years together.

The closing cards reveal information about the real Hachikō who was born in Odate in 1923. After the death of his owner Hidesaburo Ueno in 1925, Hachiko returned to the Shibuya train station the next day and every day after that for the next nine years. The final card reveals that real Hachiko died in 1934 (in fact, he died in 1935). A photo of his statue in the Shibuya train station is the last image shown before the credits roll.

The film was shot primarily in Woonsocket, Rhode Island and Bristol, Rhode Island. The newspaper reporter, Teddy, states he is from the Woonsocket Call, the daily newspaper published in Woonsocket. This is the only spoken reference to the actual location where filming took place.