While running some errands, Mitsumi - the trusted assistant of Sinnoh's renowned Pokémon Professor - comes across an uncommon sight. She spots a group of Eipam led by a boy named Hareta, who is dressed only in shorts. Hareta has possessed an extraordinary talent for making connections with Pokémon since infancy and has resided among them in the forest since then. As a result, his comprehension of the world beyond humans and their societies is limited.
When Doctor Nanakamado offers to acquaint Hareta with Dialga, a respected Pokémon god, Hareta eagerly agrees. Along with Mitsumi and a haughty Water-type Pokémon named Piplup, Hareta embarks on an exciting quest to obtain Gym Badges. However, they soon realize that they are not alone in their pursuit of Dialga's power. A mysterious group known as Ginga-dan is carrying out atrocious deeds against both humans and Pokémon, and their commanding leader is determined to take control of Dialga's ability to manipulate time.