Ash and Goh return to the Cerise Laboratory from Goh's failed attempt at catching a new Pokémon to learn that they have a video call waiting for them. The call is revealed to be from Opal, the old woman whom Ash and Goh met before in Ballonlea and who turns out to be the town's Gym Leader, much to their surprise. Opal tells them that she's hosting a tournament that she'd like the two to join. Upon learning that Leon will be participating as well, Ash immediately accepts, as does Goh, who's eager to catch some more Galar Pokémon. Chloe suddenly announces that she's coming along as well, surprising the boys. Meanwhile, Leon is traveling through Glimwood Tangle, spending some time with the local Pokémon before resuming his journey towards Ballonlea, only to realize he has no idea which way to go.
Chloe's reason for joining in is revealed as soon as the group arrives in Ballonlea, as she takes them to a local pâtisserie to enjoy some baked sweets. She explains that the secret of the delicious sweet taste is the cream, which is made by the pâtissier's Alcremie and tastes better the more the Alcremie likes its Trainer. They then notice a wild Pokémon floating over the table next to them, with Goh's Rotom Phone quickly identifying it as a Milcery. The Milcery approaches Goh in a friendly fashion, and he takes it as a sign of her wanting him to catch her. Upon receiving a confirmation that this is the case, Goh catches Milcery with a Poké Ball, before letting her back out. She approaches Goh's cupcake, but instead of the cupcake itself, she wants the Strawberry Sweet on top of it. Goh takes the sweet and places it on Milcery, who starts happily dancing in the air around him. Goh is happy to see Milcery being happy, and spins around a few times with her in his hands to celebrate making a new friend. Suddenly, Milcery unexpectedly evolves into an Alcremie, shocking the group with how quickly it happened. The pâtissier explains that Milcery evolves when it's spun around, like whisking whipped cream. Goh happily welcomes his newly evolved partner to his team.
Ash, Goh, and Chloe arrive at the Ballonlea Stadium, where Opal greets them and leads them to where the tournament will be held. Along the way, she tells Chloe that the Galarian Ponyta and Rapidash she befriended during her last visit are doing fine, making Chloe delighted. The group reaches the Gym's battlefield, but instead of Leon, they find Raihan there. Raihan recognizes Ash and wonders if Leon's absence is because he ran away. Ash protests that Leon would never run away, but Raihan says that he'd be running if it wasn't for Opal's presence, as he's never been able to disobey the old lady. Slightly intimidated now, Goh and Chloe try to back off, but Opal says they'll be participating too, much to their surprise.
Leon then arrives, apologizing for being late. With everyone present, Opal announces the tournament's name: the All-Alcremie Decoration Challenge. Each contestant will team up with one of her many Alcremie and decorate a cake with its help. Leon explains to the group that Opal likes to hold tournaments at the drop of a hat, and it's not guaranteed that they'll always involve battles. Ash is dismayed, as he was looking forward to getting to battle Leon. However, seeing the Champion's cheerful attitude, he decides to give it his all as well. Having never made a cake before, Goh is slightly nervous, but Chloe, who has made some cakes with her mother before, is excited. Each contestant goes on to choose an Alcremie to team up with: Goh chooses his own Alcremie; Chloe teams up with a Lemon Cream Alcremie; Leon invites a Matcha Cream Alcremie to compete with him; Raihan, after some hesitation, teams up with an eager Rainbow Swirl Alcremie; and Ash picks a sweet-smelling Caramel Swirl Alcremie to team up with him.
The decorating starts. Goh looks up some instructions from his Rotom Phone, while Ash gets advice from both Goh and Chloe. Raihan lets his rivalry with Leon fuel his whisking. Everyone decorates their cake their own way, with Leon and Raihan topping theirs with Berries, Chloe modeling her cake after an Alcremie, and Ash decorating his cake with whatever he thinks is nice, although Goh and Chloe don't really find Ash's cake all that appealing. Goh, unfortunately, is unable to decorate his cake at all, as Grookey ends not being able to control itself and eating all the cream on his cake.
Seeing that everyone is finished, Opal starts tasting each cake in turn. She finds something to compliment in each of them, even Goh's cake, despite it lacking any decorations. Finally, she reaches Ash's cake. She's a little bewildered by the unusual decorations on it, but tastes it anyway. Suddenly, she bursts out in excitement, finding the cake especially tasty. She then unexpectedly challenges Ash to a battle, wanting to experience the Trainer who could make his Alcremie partner produce such intriguing cream. Ash happily accepts the challenge, and Opal tells everyone to take their places.
Soon, Ash and Opal are standing at the opposite ends of the battlefield, while Leon acts as the referee and Goh, Chloe, and Raihan watch from the stands. At Leon's signal, both Trainers choose their Pokémon, with Ash choosing Pikachu and Opal sending out a Ruby Swirl Alcremie. Ash begins the battle with a Quick Attack, before following up with a series of Iron Tails. Alcremie, however, merely shrugs off the damage, surprising Ash. Opal tells Ash to look at Pikachu, causing him to realize that some of Alcremie's cream has gotten stuck on Pikachu's tail, softening the Iron Tails he used. Opal begins her counterattack by having Alcremie use Attract, infatuating Pikachu and leaving him open for a Dazzling Gleam. Pikachu manages to stand back up, however, and successfully blocks the next Dazzling Gleam with Electroweb.
Opal compliments Ash's skills before recalling Alcremie and using her Dynamax Band on her Poké Ball. She then throws the enlarged ball, sending out her Gigantamax Alcremie. Opal orders Alcremie to use G-Max Finale, causing huge projectiles of cream to fall on top of Pikachu. After a few moments, however, Pikachu starts emerging from the cream, growing bigger and bigger, until he matches Gigantamax Alcremie in size and is able to swallow an entire G-Max Finale cream projectile whole, much to Opal's shock. Thinking Pikachu has Gigantamaxed as well, Ash tells him to use G-Max Volt Crash. However, Pikachu does not attack, creating general confusion. Leon explains that this is not Gigantamax. Instead, Pikachu has simply eaten all the cream from the G-Max Move and grown massively fat as a result. Leon dubs the transformation "Feastamax", which Goh reacts to by crying out in disbelief. Seeing the situation is what it is, Opal shrugs and declares that the battle is over.
Later, all the contestants and Opal are eating the cakes made during the tournament, and they indeed find Ash's cake as delicious as Opal had described. Ash approaches Leon, promising him to battle his way through the World Coronation Series to reach and face him. Leon nods and says he'll be looking forward to it, and he and Ash share a fist bump.