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In Search of the Legendary Pokémon Dialga!! (Japanese: 神話のポケモンを求めて!! Seeking a Mythical Pokémon!!) is the first chapter of the Pokémon Diamond and Pearl Adventure! manga.

Plot
Mitsumi, the assistant of Professor Rowan, arrives at the lakeshore near Twinleaf Town having been sent to find Hareta, a boy who lives among wild Pokémon. Upon meeting him, she is astounded by how he interacts with them. She sees him climb trees with, have swimming races with , eat bark with and lift rocks with. However, the rock Hareta lifts is part of a sleeping, whose horn catches onto Mitsumi's skirt and keeps her stuck above its head. Mitsumi points out a briefcase with some Poké Balls inside to Hareta, who is fascinated by the concept of Pokémon inside balls and attempts to open one with his teeth, which results in him having a stuck in his mouth. is offended by appearing in Hareta's mouth, and refuses to battle, telling Hareta to do it himself. Hareta subsequently uses a on Onix, which results in Hareta hurting himself more than his target, however managing to calm it down anyway. As Hareta recovers, Professor Rowan arrives and takes them back to the Sandgem Town Pokémon Center.

While at the Pokémon Center, the Professor explains to Mitsumi that a good friend left Hareta in his care long ago and that after realizing Hareta's clear talent at instantly befriending Pokémon, left him to live with them in the wild. Meanwhile, Hareta attempts to befriend Piplup with no luck due to Piplup's massive pride. At this point, Professor Rowan tells Hareta about his and Mitsumi's mission: finding the Legendary God of time,. Both Hareta and Mitsumi are excited at the prospect of meeting such a Pokémon. Hareta soon gets a new outfit and Professor Rowan offers to let him switch Piplup for a more cooperative Pokémon, however, Hareta declines the offer insisting that he and Piplup will "become friends someday!" and goes outside with Piplup to train.

Unwilling to listen to Hareta, Piplup runs off on its own. However, it comes face to face with the Onix from before. Hareta soon arrives and notices that there's something strange about it this time. After being chased, they soon find Mitsumi again who tells Hareta that are strong against  and that he should use. However Piplup still refuses to listen to Hareta, who decides to take things into his own hands by jumping up and grabbing onto Onix's head, begging him to calm down. Mitsumi sends her out to battle, but before it can do anything, several wild Aipom appear in order to help Hareta. As Onix continues to flail around, it eventually heads to attack the trees. Mitsumi tells Hareta to let go to avoid getting hurt, however Hareta refuses saying that Onix is a good friend of his and that he'll never let go. Piplup is moved by this and finally steps into action, taking a direct blow from Onix. After Hareta and Piplup get up from the ground, the two of them finally work together as Piplup uses a Bubble Beam powered up by its ability. After that, Onix is finally back to its normal self again.

Sometime later, Hareta receives a Pokédex from Professor Rowan. He and Mitsumi set off on their quest to find Dialga.

Major events

 * Hareta meets Mitsumi.
 * Hareta gets his first Pokémon,, and becomes a Pokémon Trainer.
 * Something strange happens to an who Hareta is friends with, but it returns to normal after being defeated in.
 * Hareta receives a Pokédex and sets off together with Mitsumi on their quest to find.

Humans

 * Mitsumi
 * Hareta
 * Professor Rowan

Pokémon debuts

 * (inside Poké Ball)
 * (inside Poké Ball)
 * (silhouette)
 * (Mitsumi's)
 * (inside Poké Ball)
 * (inside Poké Ball)
 * (silhouette)
 * (Mitsumi's)
 * (silhouette)
 * (Mitsumi's)

Humans

 * Hareta
 * Mitsumi
 * Professor Rowan

Pokémon

 * (new)
 * (Mitsumi's)
 * (Professor Rowan's; inside Poké Ball)
 * (Professor Rowan's; inside Poké Ball)
 * (silhouette)
 * (silhouette)
 * (silhouette)
 * (silhouette)
 * (silhouette)
 * (silhouette)

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