The realization that Mitsuha died three years ago shatters Taki. He begins to lose his memories of her name, and the diary entries on his phone continue to vanish. Desperate to prove he isn't losing his mind, he treks back to the mountain crater—the family shrine—where he once went while in Mitsuha's body.
The Sacred Offering
Taki finds the hidden shrine inside the mountain. He remembers the grandmother's words about Musubi—the flow of time and the connection between souls. He finds the jar of kuchikamizake (the sake Mitsuha made) that he had left there as an offering years ago (in her timeline).
He drinks the sake, hoping it contains a piece of her soul. As he swallows, he slips on the damp floor of the cave. As he falls, he sees a vision of Mitsuha's entire life—her birth, her mother's death, her father's grief, and finally, the moment the comet struck.
One Last Switch
Taki wakes up. He's in a soft bed. He looks at his hands—they are small. He is back in Mitsuha's body! It is the morning of the festival, the day the comet is supposed to hit.
He realizes this is his final chance to save the town. He tries to convince Mitsuha's father, the Mayor, to evacuate the citizens, but his father thinks he is mentally ill. Frustrated, Taki (as Mitsuha) teams up with her friends, Tessie and Sayaka, to blow up the town's power substation and broadcast a fake emergency alert to force people to flee.
Kataware-doki (The Twilight)
Mitsuha (now in Taki's body) wakes up on the cold floor of the mountain shrine in the "future" (three years later). She sees the town is gone and realizes she is dead in his time.
Taki, sensing her presence, runs to the rim of the crater. They can hear each other's voices, but they cannot see each other because they are separated by three years of time.
Then, Kataware-doki (Twilight) arrives—the magical time of day when the blurred light allows the world of the living and the world of the dead to meet. Suddenly, they see each other. For a brief, beautiful moment, they are in their own bodies, standing on the same mountain.
Taki: "I came to see you. It wasn't easy because you were so far away."
Mitsuha: "But... how am I alive?"
The Erasure
Taki warns her about the comet and tells her the plan. Before the light fades, Taki suggests they write their names on each other's hands so they never forget.
Taki writes on Mitsuha's hand. Just as Mitsuha starts to write on his, the sun disappears. The twilight ends. Mitsuha vanishes back to her timeline, and Taki is left alone on the mountain in the future.
He screams her name, but it's already slipping away. He looks at his hand to see what she wrote... but she hadn't finished. He can't even remember why he is standing on a mountain in the middle of nowhere.
The Final Run
Back in the past, Mitsuha races down the mountain to finish the plan. She trips and falls, sliding down a dirt path. Exhausted and losing hope, she looks at her hand to remember the name of the boy who saved her.
She opens her palm. Instead of his name, Taki had written: "I love you."
With tears in her eyes and renewed strength, she stands up and heads toward her father's office one last time as the comet splits apart in the sky above.