Regarding the theme song "One more time, One more chance": What role does Masayoshi Yamazaki's song play in the montage of Episode 3? Is it merely background music, or does it become a "second protagonist" of the film, using its lyrics to complete the unspoken emotions and storylines of the characters?

This is an exceptionally precise and profound question. For 5 Centimeters per Second, the role of "One more time, One more chance" is far from being mere background music.

It can, and should, be seen as the film's "second protagonist," or more accurately, the "soul's monologue" of the entire movie. Through this MV-style montage, Makoto Shinkai achieves a genius-level fusion of narrative: he allows Masayoshi Yamazaki's voice to become the embodiment of all the emotions that Takaki, Akari, and even Kanae have suppressed for over a decade but could never put into words.

1. Role One: As the "Narrator" — Completing the Unspoken Story

The song's most direct function is to "sing" out the plot points and inner thoughts that were omitted or left as white space in the visuals.

"What more must I lose, before my heart can be forgiven?" (これ以上何を失えば心は許されるの)
- As this line is sung, the screen shows adult Takaki working numbly and a text message from his girlfriend breaking up with him. It perfectly interprets Takaki's inner turmoil: he has lost his youth, his love, and his ability to live in the present, yet he still cannot find release from his obsession with the past. He doesn't know what more he must sacrifice to forgive the version of himself who failed to protect their promise.
"I'm always searching somewhere for a trace of you." (いつでも捜しているよどっかに君の姿を)
- The scene shifts to Takaki subconsciously searching for someone in crowded train stations and at street crossings. The lyrics give this unconscious behavior a clear motive. He isn't just wandering aimlessly; he is futilely searching this vast world for that familiar figure who can no longer appear.
"If a miracle were to happen, I would fly to your side in an instant." (奇跡がもしも起こるなら今すぐ君に見せたい)
- This line, full of powerless fantasy, stands in stark contrast to the image of Takaki's lonely figure. It voices his deepest desire: he wishes for a "miracle" to shatter this damned reality, allowing him to return to the past or to appear before Akari right now. However, the visuals tell us that a miracle never happens.

The lyrics of this song are like a script that Shinkai wrote but never had the characters speak. It allows the audience to dive into the characters' subconscious and hear their truest, most painful cries.

2. Role Two: As the "Connector" — Weaving Together Fragments of Three Timelines

The brilliance of this montage lies in how it shatters the linear narrative, intertwining the past, present, and imagination. And "One more time, One more chance" is the "thread" that ties these temporal fragments together.

Emotional Synchronicity: As the song plays, the visuals rapidly switch between 13-year-old Takaki, high-school Takaki, and adult Takaki. The song makes the audience understand that no matter how much time has passed or how his appearance has changed, Takaki's core emotion has never wavered. The song becomes the "theme music" for his decade-long emotional state.
Interwoven Perspectives: The montage features not only Takaki but also intersperses clips of Akari putting on her ring and Kanae crying by the sea. At this moment, the song transcends Takaki's personal story to become a shared lament for all three about "missed connections" and "regret." It suggests that even Akari and Kanae, who have seemingly "moved on," still hold complex feelings for that period in their lives deep within their hearts.

3. Role Three: As the "Emotional Amplifier" — Pushing Emotions to Their Climax

Music itself possesses a powerful emotional resonance. Masayoshi Yamazaki's slightly hoarse, world-weary voice, combined with the melody's repetitive, layered structure, creates an unparalleled emotional atmosphere.

From Repression to Eruption: The first half of the song is relatively calm, paired with mundane, numb visuals to build a repressive atmosphere. When the song reaches its chorus and the emotions fully erupt, the visuals flash back to the most crucial and beautiful memories (like the kiss under the cherry blossom tree). This synchronized audio-visual explosion causes the audience's long-accumulated emotions to burst forth in an instant.
Defining the Film's "Undertone": The song itself was created in 1997, a decade before the film. It is, in its own right, a classic story of loss and searching. By choosing this song, Shinkai effectively pre-set a sad, nostalgic, yet incredibly affectionate emotional "undertone" for his film. While watching, the audience is unconsciously guided by the song's mood.

Conclusion: The Soul of the Film, Not an Accessory

Therefore, "One more time, One more chance" is by no means just BGM. Makoto Shinkai made a bold choice: he handed over the interpretive power of the film's most critical emotional climax and ending almost entirely to this song.

It isn't "accompanying" the visuals; it is "dialoguing with" and even "leading" them. It acts like an omniscient narrator, standing at the end of time, looking back at the fates of these three young people scattered by the years, and then, for them and for the entire audience, singing this most magnificent and heartbreaking requiem for youth.

It can be said that without this montage, 5 Centimeters per Second would not have its final, nuclear-level emotional impact. This song is the longest and most painful love letter that Makoto Shinkai wrote for all his characters—one that was ultimately never sent.