In Pallet Town, a young Ash Ketchum is beside himself with anticipation—he’s going to Professor Oak’s Pokémon Camp! But when the day arrives, he oversleeps, and Professor Oak and the other campers leave without him. Two of the young campers, Goh and Chloe, become separated from the group and encounter the Mythical Pokémon Mew—and Goh vows to catch it someday!
Meanwhile, a once-lonely Pichu is spending its days with a group of friendly Kangaskhan, but when the time comes for it to set off on its own, it suddenly evolves into Pikachu! After some time, Pikachu ends up at Professor Oak’s lab…and the rest is history!
Four years prior to when he began his Pokémon journey, a six-year-old Ash rushes downstairs to show his mother a poster advertising Professor Oak's Pokémon camp. Seeing her son's enthusiasm, Delia agrees to sign him up, but she admits he will have to get himself up since she has things to do. However, Ash is too excited to listen. Meanwhile, in the forested outskirts of Pallet Town, a young Pichu observes his surroundings before being attacked by a Ekans, though Pichu uses Thunder Shock to defend himself. Ekans slithers away, while the electrical discharge leaves Pichu exhausted.
Pichu quickly recovers and quickly notices, unlike all the other wild Pokémon, that he is alone. Just then, a group of Koffing floats by and unleashes an odor, leaving Pichu so disoriented that he falls over a cliff. However, he is caught by a Kangaskhan. Pichu is initially frightened by the large Kangaskhan mother, but he quickly takes a liking to her and her child. As the Kangaskhan herd prepares to leave, the mother places Pichu in her pouch. The herd later stops by some Oran Berry trees to eat, with both the Kangaskhan baby and Pichu each getting a Berry to enjoy. Having finally found a place where he belongs, Pichu continues to live with the Kangaskhan.
Several weeks later, Professor Oak's camp begins. While many of the attendees including Chloe and her friend Goh have arrived on time Ash is still sleeping as, in his excitment, has broken his alarm clock. Goh and Chloe are eager to begin their field trip, so Professor Oak decides to set off and stop waiting for Ash. Ash eventually wakes up, and in a panic, he rushes out of home still wearing his pajamas desperate to make it to the lab before everyone leaves. During the forest excursion, the kids and Oak observe a young Trainer catching herself a Caterpie. Throughout the day, Goh continues to showcase his Pokémon knowledge, identifying every Pokémon in sight and reciting facts about them. While the other kids are impressed, Chloe orders Goh to allow Professor Oak to do the talking instead. However, Goh continues to provide his input, even correcting Oak on his statement that all Pokémon evolve.
While Oak leads the group over a bridge, Goh and Chloe lag behind. Chloe tells Goh that he should focus on making friends instead, but Goh insists that he wants to find a rare and never-before-seen Pokémon. Chloe dismisses Goh's ambition as a dream, but Goh asserts that it is the future. The cry of a Nidoking attracts their attention, and Goh hones in on Nidoking's opponent, a Mew. Nidoking uses Earth Power, but Mew simply Teleports to evade. Mew replies with the same move, which easily hits the Nidoking. As Nidoking attempts a Sludge Wave, Mew Teleports away again before retaliating with the same move once again. Nidoking becomes frustrated and launches a Fire Blast at his shifty opponent, though Mew again evades and then proceeds to defeat Nidoking with its own Fire Blast. Chloe and Goh are impressed by the mysterious Pokémon, and they give chase as it flies away. In their search, the pair come across a Magmar, though Mew quickly Transforms to its regular form to reveal itself. Mew leaves yet again with Goh and Chloe in pursuit, transforming into a Tauros to run across the land before changing into a Lapras to travel downstream.
Elsewhere, Pichu continues to enjoy his time with the Kangaskhan family. He plays a game of hide-and-seek, but is quickly spotted by the Kangaskhan baby, ending the round. While searching for Pichu again, the young Kangaskhan spots a Lapras as it cruises down the river. Lapras reveals itself to be Mew, who quickly takes interest in the Kangaskhan baby. Mew overhears Goh and Chloe’s voices and flies away, though the Kangaskhan baby scales some ledges to follow its mysterious encounter. Meanwhile, the Kangaskhan mothers call their babies out for a Chansey egg meal. Pichu and the Kangaskhan mother soon notice the infant's absence and begin to search for it. The search party makes their way over to the stream, where they overhear the young Kangaskhan's cries. While chasing after Mew, the ledge underfoot gives way, sending the young Kangaskhan hurtling towards the water below. Chloe and Goh stop as they hear the young one's screams. Mew turns back and promptly uses Psychic to save the Kangaskhan baby from harm, and returning her to her mother’s arms. With the family united, Mew soars away. Afterwards, Professor Oak rushes towards the two kids, relieved to have finally found them. While returning to the rest of the group, Oak speculates they encountered a Mew, a Mythical Pokémon believed to contain the genes of all Pokémon. Hearing this, Goh sets his sights on catching Mew one day. Ash, meanwhile, sits on his bed at home, miserable at having missed the camp entirely.
Some time passes, and both Pichu and the Kangaskhan baby have grown. The Kangaskhan herd seeks refuge from a rainstorm in a cave, but Pichu notices his adoptive mother is having trouble carrying both of them at once now. He tries to jump out to lighten her load, but the mother Kangaskhan picks him back up. As night falls, Pichu knows it is time to move on. While the Kangaskhan are fast asleep, Pichu leaves his adoptive mother's pouch, takes a moment to observe his family as they sleep, and tearfully ventures outside. Grateful for their kindness and company, Pichu suddenly evolves into a Pikachu. Looking in a nearby puddle, Pikachu is initially startled by his reflection before twirling around in excitement. Pikachu waves goodbye to his adoptive family and departs.
Four years later, Professor Oak gives this very Pikachu to Ash, who is now an aspiring young Trainer. Pikachu, however, isn't happy with Ash's enthusiasm and shocks him with Thunder Shock. However, as time passes, Ash and Pikachu become close friends. In the present day Vermilion City, Goh is pumped up to realize his dream.