In the Vermilion City Pokémon Center, Nurse Joy and Chansey go into a back room, which contains a transparent case with a Pokémon Egg; machinery is keeping track of its activity. Upon noticing there have not been any changes in activity, Nurse Joy wonders why the Egg is refusing to hatch. Meanwhile, at the Cerise Laboratory, Professor Cerise tells Ash and Goh how Professor Elm was the first person to make the existence of Pokémon Eggs known, and then he remarks how no one has ever even seen a Pokémon Egg being laid, making their origin a mystery. Ash recalls that he has never seen an Egg getting laid either, and Goh is impressed to learn that Ash has actually seen an Egg before. This inspires Goh to look for an Egg as well, and he leaves to search for one with Ash, undeterred by Ren mentioning how finding an Egg won't be easy.
While walking through a local park, Goh spots what he initially thinks is an Egg, but it quickly turns out to be an Exeggcute. After scanning it with his Rotom Phone, Goh throws a Poké Ball at the Egg Pokémon and successfully catches it. Just then, Ash's Rotom Phone goes off, alerting him that another World Coronation Series participant, a Trainer named Hayden from Fuchsia City, who's ranked as the 1,024th, is nearby and wants to challenge him. Hayden quickly comes into their view, and the Drone Rotom flies in, ready to referee the match. At Rotom's mark, both Trainers send out their Pokémon, with Ash choosing Pikachu and Hayden going with Tauros. Based on the species's Pokédex description, Goh is able to deduce that this Tauros must've been the boss of its herd. As Pikachu's Iron Tail collides with Tauros's Horn Attack, invisible Aura waves are sent out, and the Egg at the Pokémon Center reacts to them. The battle goes on, until Ash has Pikachu use Thunderbolt to finish the match, he radiates out Aura waves too, and the Egg starts vibrating, which doesn't go unnoticed by Chansey.
The match ends with Ash as the victor, although Pikachu did take some damage from Tauros's Iron Head. Just as Ash tells Pikachu they should head to the Pokémon Center, Ash senses something. Goh goes to congratulate him but quickly notices his absent-mindedness. Ash tells that he feels like someone is calling him. Goh is confused, as there is no one around. He remembers how Ash claimed to be able to hear Lugia's voice, but Ash says that this is different. Hayden congratulates Ash on the good battle, and the Drone Rotom flies away to update the World Coronation Series rankings. At the Pokémon Center, the Egg settles down, but Nurse Joy notices the signs of its activity and is surprised.
Ash and Goh head to the Pokémon Center, with Ash rushing ahead without even realizing it. They arrive at their destination, and while Pikachu is being treated, Ash follows the strange sensation to a back room, where he finds the case with the Pokémon Egg inside. Goh and Nurse Joy follow him, with the former being surprised at seeing the Egg. Ash asks why the Egg has wires attached to it, but before Nurse Joy can explain more than that she had received it from a Day Care worker, the Egg suddenly breaks free from the wires and rolls towards Ash. Seeing this, Nurse Joy suggests that Ash should take the Egg and look after it. She explains that the Egg was originally found in the Sinnoh region, and has been passed down from one Day Care worker to another, but it has simply refused to hatch. Ash happily introduces himself to the Egg and urges it to hatch so it can experience the fun activities outside.
Later, at the Cerise Laboratory, Chloe wonders what Pokémon is inside the Egg, but Professor Cerise tells her that there's no way to be sure until it hatches. Ren and Chrysa explain how an Egg needs to spend time with healthy Pokémon to hatch, so Ash tells the Egg that they're going for a stroll. Goh says that the Egg can't hear him, but Professor Cerise tells him a tale of how a Jigglypuff had sung to an Egg every day, and when the Egg had eventually hatched into an Azurill, it had sung the same song as Jigglypuff had, indicating that Pokémon can still hear things while inside their Eggs. Hearing this, Goh quickly introduces himself to the Egg and apologizes for doubting its ability to hear him. They take a trip to the Cerise Park, where Ash introduces the Egg to his Dragonite. He stops Dragonite from hugging the fragile Egg, so it decides to hug Pikachu instead.
That night, while Ash and Goh sleep, the Egg begins to glow, which alerts Pikachu, who, in turn, wakes up the others. Ash and Goh rush to the Egg as it begins to crack and glow even more intensively. As the glow dies down, they see that the newly hatched Pokémon is a Riolu. Goh checks it with his Rotom Phone, which mentions that a Riolu is able to crest three mountains and cross two valleys in a single night. Ash and Goh introduce themselves to Riolu, but it just looks around, looking for the source of the Aura it had sensed earlier. When Ash reaches out to it and tries asking it what's wrong, Riolu uses Vacuum Wave on him, sending him flying back onto his bed. As Ash gets up and compliments Riolu's strength, it suddenly jumps out of the window and rushes away.
Riolu comes across a pond of Poliwrath, Poliwhirl, and Poliwag, and tries attacking the Poliwrath, but is completely outmatched, and gets counterattacked. Meanwhile, Ash and Goh are looking for Riolu in the woods. Goh is pessimistic about their chances of finding it, given what his Pokédex had told about its stamina, but Ash, knowing that Riolu still doesn't know much about the world, is determined to find it. He admits that the call he had heard the day before had been Riolu calling out to him, and hearing Nurse Joy's description of the Egg had made him feel that Riolu had been waiting for him this whole time, although he now thinks he was wrong. Goh suggests they split up to cover more ground, and Ash agrees.
As Ash and Goh search, Riolu keeps attacking every wild Pokémon it comes across, getting attacked back in response. It eventually comes to a rocky cliff-side and starts climbing it. However, it soon turns out Riolu climbed onto an Onix, who tosses it onto a cliff, angered by the disturbance. Riolu starts fighting against the Rock Snake Pokémon, but its attacks do little against the tall Rock/Ground Pokémon. As Riolu is hit by Onix's Dragon Tail, Ash, having heard Onix's cry and followed it, arrives to help Riolu. As he and Pikachu prepare for battle, he starts radiating powerful Aura waves, which Riolu recognizes as the ones it had sensed before. Pikachu used Iron Tail on Onix, but before he can attack again, Riolu pushes him out of the way and demands to get to battle instead. He accepts and tells Riolu to use Vacuum Wave, stopping Onix's attack. Riolu then tackles Onix, toppling it over, and as Riolu prepares to attack with another Vacuum Wave, Onix finally flees. Ash calls Riolu to stop its attack and compliments it. Riolu stumbles, and Ash offers to carry it to the Pokémon Center, but it stubbornly refuses, choosing to walk the way there instead.
Later, once Riolu's treatment at the Pokémon Center is complete, it jumps off the medical table and tries to tell Ash something. Nurse Joy tells Ash that Riolu is probably asking to be caught by him, so, upon receiving a confirmation from Riolu that this is the case, Ash tosses a Poké Ball at Riolu and catches it, making a new addition to his team. He then lets it out, and Riolu proudly takes the lead as the group leaves the Pokémon Center.