Goh joins his parents, Camille and Ikuo, on a family vacation to Azalea Town in the Johto region. Along the drive to their cabin, Goh's mood changes when he sees a silhouette which triggers a flashback of himself walking in the rain with a Sunflora. The cottage manager greets the family, remarking that it has been some time since they last met, and Goh adds that it has been three years now. The manager notices that Goh has really grown up now, with Camille adding that he is now ten years old and a Pokémon Trainer. The cottage manager then hands Ikuo the keys and leaves the family to enjoy their vacation. Once inside, his parents instantly crash and fall asleep, so Goh decides that he will head off into nearby Ilex Forest and catch himself some Johto Pokémon.
As Goh begins, a familiar sound, a Hoothoot cry, triggers another flashback from three years ago. The then seven year old followed the Hoothoot deep into the forest, where he finds himself surrounded by Pokémon. He then sets his sights on finding Mew, soon chasing after something elusive and agile. As he leaps to grab it, another young boy does the same, with the pair coming face to face and missing their target. While the Pokémon turns out to be a tricky Misdreavus, Goh asks the boy why he shouted "Celebi" just before. The boy is quick to point out that Goh mistook it for a Mew, before admitting that it appears they are both in the same boat. He introduces himself as Tokio from Cianwood City, quickly becoming friends with Goh. Goh brags that he has actually seen a Mew during his time at Professor Oak's camp, impressing Tokio. As Goh continues to rattle on about Mew, Tokio already knows a lot about it as well, but is nonetheless impressed by Goh's knowledge. Goh is taken aback when Tokio asks if he knows anything about Celebi, and quickly turns the tables to cover up the fact that he knows little about it. Tokio says that Celebi is a forest guardian, appears in only beautiful forests, and are capable of time travel. Learning that Tokio is yet to encounter Celebi, Goh offers to help him in his search.
Together, Tokio and Goh search Ilex Forest for the elusive Celebi. Their search, however, is proving difficult. They stop by a waterfall for a moment, where Goh urges Tokio to remain confident and pulls out a handheld computer, built by his parents, to help them find Celebi. Goh notices a trend after scouring through sighting reports, and suggests they travel to the Ilex Forest shrine. Goh checks the map, which shows that the shrine should be behind the waterfall and cave system somewhere. Afterwards, the boys find themselves surrounded by tall grass. Goh rushes ahead, with Tokio following, only to encounter a wild Ariados, which sends them running back to where they started. Goh and Tokio press forward again, only to find an agitated Pineco.
Goh comes up with a new plan and rushes back leaving Tokio by himself. Goh soon returns with a Sunflora to assist, revealing that he borrowed it from the cottage manager. Tokio sees the merit in Goh's plan but takes a moment to scold his friend for leaving him alone for so long. Goh apologizes and says that he would never break a promise. The boys resume their search, with Sunflora using Razor Leaf to clear a path. They press ahead until evening, coming to a stop at a clearing because Sunflora gets its energy from the sun and is only awake during the daylight hours. As the night sky sets in, Tokio and Goh admire the stars, which reminds them of how vast the universe is. Noticing that Tokio is feeling cold, Goh decides they best head back. They reach the roadside and part ways, promising to meet up the next day and explore more of Ilex Forest.
The next morning, Goh rushes out of the holiday cabin, and returns to the clearing with Sunflora. He arrives there, only to find Tokio isn't anywhere to be seen. As time ticks by and rain begins to pour, Goh becomes increasingly frustrated that Tokio never shows up. He and Sunflora leave the forest, where they are found by Goh's parents completely drenched from the storm. In the present, Goh recounts that was the first and last time he saw Tokio.
Goh's Raboot and Sobble turn to their Trainer, but Goh assures them he is fine and returns his attention to catching Pokémon. He calls out his Scyther and Farfetch'd to help cut down the tall grass. Scyther's Slash attack uncovers an Ariados, which Goh catches with a single throw. Next, they encounter a Pineco, and Goh adds it to his party. The two Pokémon remind Goh of his time with Tokio, leaving him feeling frustrated at the betrayal of trust. Pressing ahead, Goh finally arrives at the Ilex Forest shrine. He spots a letter poking out, and opens it up to read it, revealing that it from Tokio. In it, Tokio apologizes for not keeping his promise, admitting he had a fever on the day. He returned the day after, but Goh was nowhere to be seen. Tokio wrote that he then returned the next year, discovering the shrine in the hopes that Celebi could send him back in time to help him keep his promise. After Celebi never answered his pleas, Tokio left the letter at the shrine instead. A voice calls out "Goh", leading Goh to turn around. Much to his shock, Tokio, and his Pokémon partner Chikorita, arrive at the shrine. Tokio admits he has so much to tell him, and the two boys begin walking over to greet each other. Meanwhile, a pair of Celebi, including a Shiny one, fly over them before leaving.
