Plot
The ten-year-old aspiring Pokémon Trainer Ash Ketchum wakes up late one morning after having broken his alarm clock in his sleep. He eventually makes it to Professor Oak's lab, but he is told by Oak the three starters Pokémon (Bulbasaur, Squirtle, and Charmander) have already been taken by Pokémon Trainers who were on time. However, Oak reveals he has one more Pokémon, which is a Pikachu. Despite Pikachu's volatile and feisty personality, as well as his refusal to get inside a Poké Ball, Ash happily takes Pikachu on his journey.
Outside of Pallet Town, Ash encounters a wild Pidgey. When Pikachu refuses to fight it, Ash tosses a rock at it, but it hits a Spearow that mistakes Pikachu as the culprit. Soon, more Spearow shows up forcing Ash and Pikachu to run. Injured and tired, Ash tells Pikachu to get inside his Ball, but he once again refuses. Instead, Pikachu defends Ash from the Spearow flock by summoning a large Thunderbolt that scares them away. As the storm ceases, Ho-Oh flies above and drops a feather called a Rainbow Wing. Ash and Pikachu decide to go and find the Legendary Pokémon.
After hearing the Legendary Pokémon Entei is nearby, Ash and several other Trainers rush into the woods to look for it. Ash finds the Entei but ends up fighting over it with a young girl Trainer named Verity and an aspiring young boy Pokémon Professor named Sorrel. When Entei escapes, Ash and Verity argue and Sorrel leaves, urging them to find shelter. As they look for shelter, Ash and Verity find a Charmander, which they quickly learn belongs to an arrogant Trainer named Cross who left it out in the rain.
Cross refuses to take Charmander back, calling it weak, so Ash and Verity come across a cave where Sorrel happens to be and help Charmander back to health. Sorrel reveals that Ho-Oh only gives the Rainbow Wing to the "Rainbow Hero" who is destined to fight it, as a shadow Pokémon named Marshadow watches. The next morning, Ash convinces Charmander to join them. The group follows the Rainbow Wing, which guides them toward the Raizen Mountain Range, to meet Ho-Oh. Charmander evolves into Charmeleon. Verity sees a Suicune at a nearby lake. Cross returns and challenges Ash's Charmeleon with his Incineroar. Charmeleon is savagely defeated, causing Ash much grief.
Upset over his loss to Cross, Ash openly admits that he could have won with Pikachu and wishes he had gotten Bulbasaur and Squirtle with him. Alone in the woods, Marshadow puts Ash in a sleep-like state as the Rainbow Wing turns dark. Ash has a dream where no Pokémon exist and he attends regular school with Verity and Sorrel. Realizing something is missing, he remembers Pikachu and wakes up to his friends, and the Wing regains its color. Ash then reunites with his friends and apologizes for his actions, especially Charmeleon. Sorrel then explains how he lost his Luxray after a snowstorm which results from Sorrel afraid to befriend Pokemon ever since that experience. During the conversation, Pikachu notices Marshadow and uses his Thunderbolt but accidentally wakes up a bunch of wild Primeapes. Luckily, Ash used his Metapod to get the Primeapes webbed and as a result, Metapod evolves into Butterfree and drives the Primeapes to sleep. The next morning, they see a Raikou after Ash releases his Butterfree to mate and migrate with a Pink Butterfree. The group realizes they are getting close when the Wing begins to glow in the direction of Mount Tensei.
On the summit, the group makes it to a crystal-like structure called Rainbow Rock but is interrupted by Cross who wants to fight Ho-Oh himself, having saw Ho-Oh too, but did not receive the Wing. Cross' Incineroar fights Ash's Charmeleon which quickly evolves into Charizard and defeats Incineroar in a Flamethrower clash. Cross, refusing to accept his loss, grabs the Wing and places it on the crystal, but due to his evil heart, it turns dark, causing Marshadow, an agent of Ho-Oh's, to turn the local Pokémon evil and attack Ash and his friends. The Trainers fight back but are overwhelmed by the severity of the situation. Ash finally withdraws a severely weakened Pikachu into his Poké Ball as the brainwashed Pokémon kills him, and he fades away as the Wing disintegrates. Pikachu comes out of his Poké Ball, realizing his best friend died. Heartbroken and devastated, he unleashes a massive Thunderbolt which hits Marshadow and its army, releasing them from Rainbow Wing's corruption.
Ash finds himself in an alternative dimension where Pokémon do not exist, but manages to find his way to back to life. Ash takes the newly formed Rainbow Wing and places it on the crystal structure. Ho-Oh arrives, healing everyone and Ash challenges it with Pikachu to battle (although the outcome is never shown). Cross leaves on good terms and eventually Verity and Sorrel leave Ash to achieve their own dreams.
Verity returns to her hometown to see her mother; Sorrel and his Lucario spot an Articuno which he wanted to study; Cross travels peacefully with his Pokémon; and Ash and Pikachu travel on their own while being tailed by Team Rocket. Ho-Oh drops a Rainbow Wing as it flies across the sky.
