Ash and Professor Kukui find the wild Rockruff who lives with them limping home, covered in scratches and burns, completely exhausted. The next night, they follow it to Clawmark Hill, where two Lycanroc keep watch over the wild Pokémon who come together to battle and train. Rockruff battles a fearsome Magmar but is defeated, and Ash offers to help it learn a new move, Rock Throw.
After some serious training with Ash and Pikachu, Rockruff returns to challenge Magmar to a rematch—and this time, it wins! After seeing how well Ash and Rockruff work together, Professor Kukui encourages him to catch it—and both Pikachu and Rockruff enthusiastically agree!
When they return home one night, Ash, Pikachu, Rotom, and Professor Kukui notice that Rockruff is missing. Rockruff soon returns, injured. Professor Kukui heals Rockruff with a Potion after taking a look at its injuries. Ash wonders if Rockruff had gotten into a fight.
The next morning, Ash explains the strange situation with Rockruff's injuries to his classmates. To their surprise, Ash reveals that Rockruff doesn't actually belong to Professor Kukui, then explains how Kukui and Rockruff met. Sophocles mentions that Rockruff could be close to evolving. Rotom provides information on Rockruff's evolved form, Lycanroc, plus its Midday and Midnight Forms. Rotom also informs Ash that when Rockruff is close to evolving, it might become more aggressive and independent.
That night, Ash returns home, and Rockruff greets him happily. Professor Kukui had returned home before Ash because of how worried he was about Rockruff. He shows Ash the TV, where footage from several CCTV cameras are on display. Professor Kukui explains he set cameras up all around the room, but he is worried about Rockruff going outside given the previous experience with Rockruff coming home injured. Ash wonders what Rockruff does when outside the lab. Ash and Professor Kukui head out of the lab, leaving Rockruff to guard the house. However, they don't go far, opting instead to hide behind a bush. Professor Kukui pulls up the footage on his phone, and on the screen, they see Rockruff attacking a stuffed toy that Kukui left for it. Professor Kukui is puzzled, noting that hasn't happened before. At that moment, Rockruff leaves the house. Professor Kukui and Ash proceed to follow it through the forest.
Up ahead, two Lycanroc – one a Midday Form and the other a Midnight Form – call out. Many Pokémon respond to the call by approaching them, including Rockruff. Ash and Professor Kukui watch from a distance as Ash points out both of the Lycanroc. Professor Kukui explains that this is Clawmark Hill. All of the Pokémon gathered watch as the two Lycanroc face off against each other. After a brief battle between the Lycanroc, all of the Pokémon start testing their strength against each other. As Ash watches, he begins to understand the current situation. He tries to approach Rockruff, but Professor Kukui stops him, since they need to know what Rockruff's motivations are. A Magmar is then seen battling several opponents at once before being challenged by Rockruff. Rockruff and Magmar start to battle as Ash watches, worried. Rockruff prepares to launch an attack, but is hit by Flamethrower before it can execute the move. The battle continues, but Rockruff is soon knocked out.
Magmar celebrates its victory before being challenged by a Pangoro. All of this makes Ash, Rotom, and Professor Kukui wonder why Rockruff is coming here. Later, Rockruff is heading home when it comes across Ash and Professor Kukui and greets them happily. Ash tells Rockruff that it was amazing taking on all of those strong Pokémon and offers to do some training with it so it can learn a new move. Rockruff agrees to this, and Pikachu agrees to help. The next day, down on the beach, Professor Kukui guesses that Rockruff was attempting to learn Rock Throw. Ash starts telling Rockruff to concentrate its power and starts making a few moves, which Pikachu imitates and powers up an Electro Ball. Ash and Pikachu start doing a few exercises on the beach and Rockruff watches Pikachu's movements. Later, Kiawe has Turtonator use Flamethrower on Pikachu which Pikachu dodges. Ash tells Rockruff that that's all it has to do when attacked with Flamethrower.
The training continues as Ash has Rockruff power up to use Rock Throw. Rockruff fires off the Rock Throw, which hits Ash as planned, and Ash compliments Rockruff on doing great. His classmates comment that it must have hurt. The training continues with Pikachu and Rockruff having a battle, in which Rockruff is able to hit Pikachu with Rock Throw. That night, Rockruff becomes more active and Professor Kukui realizes that it is time for battling. They then make their way back to Clawmark Hill. Ash, Professor Kukui, Rotom, and Pikachu watch from a distance as Rockruff goes over to Magmar to indicate that it wants to battle. Magmar accepts, and the battle starts with Rockruff managing to dodge Flamethrower. Rockruff is able to attack Magmar but Magmar attacks back and sends Rockruff flying. Rockruff dodges another Flamethrower and gets ready to use Rock Throw, only to be hit by another Flamethrower. Rockruff doesn't give up and runs towards Magmar, who runs towards Rockruff and, after a few more hits, Magmar is defeated.
Down on the beach, Professor Kukui suggests to Ash that this could be a good time for him to catch Rockruff. Professor Kukui says Rockruff has more trust in Ash than anyone else as, thanks to Ash, Rockruff was able to master Rock Throw. This makes Ash the perfect choice for Rockruff's trainer. With Pikachu and Rockruff both agreeing, Ash gets out a Poké Ball and catches Rockruff. Ash calls Rockruff back out and they continue with their training. Ash calls it part 2, which confuses Rotom as it is the same as part 1, but Professor Kukui says that's just Ash's style and they both join them.