Mitsumi, the assistant of Professor Rowan, arrives at thelakeshore 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, seeing him climb trees with Aipom, have swimming races with Magikarp, eat bark with Bidoof and lift rocks with Geodude. However, the rock Hareta lifts is part of a sleeping Onix, 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 Piplup stuck in his mouth. Piplup is offended by appearing in Hareta's mouth, and refuses to battle, telling Hareta to do it himself. Hareta subsequently uses a headbutt on Onix, which does more damage to Hareta than the target, however manages to calm it down anyway. As Hareta recovers, Professor Rowan arrives and takes them back to theSandgem Town Pokémon Center.
When 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, to find the Legendary God of time, Dialga. 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 insisting that he and Piplup will "become friends someday!" and goes outside with Piplup to train.
Not wanting to listen to Hareta, Piplup runs off on its own. However, it comes face to face with the Onix from before, and 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 Water-type are strong against Rock-type and that he should use BubbleBeam. 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 Starly out to battle, but before it can do anything the wild Aipom appear 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. As Hareta and Piplup get up from the ground, the two of them finally work together as Piplup uses a BubbleBeam powered up by itsTorrent ability. After this, Onix is finally back to its normal self again.
Some time later, Hareta receives a Pokédex from Professor Rowan as he and Mitsumi set off on their quest to find Dialga.
