The film's plot tells the story of how Satoshi (Ash) and Pikachu came to know each other. On the day they set out from Masara Town (Pallet Town), both of them saw a Ho-oh flying and made a vow to someday meet it. 

(Source: Anime News Network, edited)

A young boy named Satoshi embarks on a journey to become a "Pokémon Master" with his first Pokémon, Pikachu. Joining him on his travels are Takeshi, a girl-obsessed Rock Pokemon Trainer, and Kasumi, a tomboyish Water Pokémon Trainer who may have a crush on him. Satoshi and Co. end up traveling through various regions, including Kanto, the Orange Islands, and Johto, and then enter the Pokémon League competitions there. Along the way, they run into many confrontations with Musashi, Kojirou, and Nyarth, a trio of Pokémon thieves who are apart of an evil organization called "Team Rocket". But everytime Team Rocket try to do their evil deeds, they fail thanks to Satoshi and his Pokémon. 
(Source: Anime News Network)

When Satoshi oversleeps on his 10th birthday, he ends up with a stubborn Pikachu instead of the first partner Pokémon he wanted! But after a rocky start, Satoshi and Pikachu become close friends and true partners - and when they catch a rare glimpse of the Legendary Pokémon Ho-Oh in flight, they make plants to seek it out together.
Trainers Makoto and Souji join Satoshi on his journey, and along the way, they meet the mysterious Mythical Pokémon Marshadow. When they near their goal, an arrogant Trainer named Cross stands in their way. Can Satoshi and Pikachu defeat him and reach Ho-Oh as they promised, or will their journey end here?
 
(Source: Viz Media)

A compilation of new stories of the Pocket Monsters: Kimi ni Kimeta! movie, telling the encounter between Satoshi and Pikachu, the story of Marshadow and Ho-Oh.
 
1. Satoshi & Pikachu: Beginning (サトシ＆ピカチュウ ビギニング); CoroCoro Comic 2017-05 
2. Moeyo Marshadow (燃えよマーシャドー); CoroCoro Comic 2017-07 
3. Niji no Yuusha (虹の勇者); CoroCoro Comic 2017-08 
4. Pikachu to Satoshi no Boushi (ピカチュウとサトシのぼうし); kakioroshi

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