Spoiler warning: this article may contain major plot or ending details.
After interacting with a decoration at Peachy's, the player receives a phone call from Arven, asking if he can visit the player at their house at Cabo Poco. During the conversation, the decoration disappears from behind the player's back. Upon returning home, the player meets Arven, along with Nemona and Penny, who had overheard his phone call and insisted on coming along. The player's Mom gives the player a letter from Kieran, informing them that he's taking a break from Blueberry Academy and is currently residing at his home in Mossui Town. The letter also includes tickets for the player and their friends, who are all feeling happy about the idea of getting to visit Kitakami, so they depart there immediately.
In Mossui Town, Nemona, having learned that Kieran used to be the Champion of Blueberry Academy, is quick to challenge him to a battle, and defeats him soundly before the player even has a chance to come see their match. Just then, Carmine shows up, performing a weird dance and repeating the word "mochi" over and over. Kieran explains that she's been acting like this ever since he sent his letter to the player. He suspects that his sister's odd behavior might have something to do with a curse that's rumored to be going around, but the group encourages him to stay positive. The group retreats to the Community Center for the night, but while they search for a hidden TV remote, Nemona suddenly disappears. When the player and Kieran go look for her, they find more villagers acting in the same strange way as Carmine. Spotting Nemona, they follow her to Kitakami Hall, where they witness Kieran's grandfather Yukito falling victim to the strange behavior as well after eating some suspicious mochi offered by his already-possessed wife.
The player and Kieran return to Mossui Town, where a small purple Pokémon also posseses Arven and Penny by firing pieces of mochi directly into their mouths. After defeating them in a Multi Battle, the player and Kieran pursue the mysterious Pokémon to Loyalty Plaza, where they also find Nemona, who has also been possessed. With a horde of possessed villagers approaching them, Kieran holds them off while the player battles Nemona. Once she has been defeated, the player challenges the mysterious Pokémon, identified as the Mythical PokémonPecharunt, defeating and catching it. This releases everyone from Pecharunt's control, with only vague memories of what they did while possessed. After spending some more peaceful time in Kitakami, the player and their friends return to Paldea, while Kieran and Carmine return to Blueberry Academy.
After following Nemona as she heads towards Kitakami Hall, the player and Kieran are stopped by Kieran and Carmine's grandparents Yukito and Hideko. Hideko offers toxic mochi to Yukito, who eats it and becomes possessed, and the two of them engage the player and Kieran in a Multi Battle.
Arven and Penny engage the player and Kieran in a Multi Battle after Pecharunt launches toxic mochi into their mouths, causing them to fall under its influence.
This section is incomplete. Please feel free to edit this section to add missing information and complete it. Reason: What happens if the player loses here?
The player is required to battle Pecharunt after defeating Nemona whilst she is under its influence. It must be caught upon its defeat, and is coded to never be Shiny. It has set Timid Nature when caught.
When talking with and battling against characters under the effect of Pecharunt
Mochi MOCHI!!!*
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Unknown
When the group discusses Carmine under Pecharunt's control
A Curse?!*
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When battling Pecharunt
Battle! (Pecharunt)!*
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Unknown
Song names marked with an asterisk are unofficial, as they have not received a soundtrack release.
Trivia
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Although it is never shown to the player in-game, all of the Pokémon owned by Aya, Shige, Yukito, and Hideko are programmed to have Poison as their Tera Type, likely as a reference to the effects of Pecharunt's toxic mochi.
In the Japanese version, the possessed shout out "Kibi kibi!" instead, referring to millet dango from the folktale Momotarō.
If the player uses Ogerpon in the battle against Pecharunt, Ogerpon will react with anger, yelling out "GRAH! Pon PON!!!" at it, likely for what it did to Ogerpon and her companion in the past.
If the player uses a member of the Loyal Three; Okidogi, Munkidori or Fezandipiti, in the battle against Pecharunt, Pecharunt will react with shock and anger, yelling out "Pechaaa! Chraaahhh!" at it, likely for the betrayal as Pecharunt's retainer.
Pecharunt will repeat this reaction every time a member of the Loyal Three is sent out for the first time by the player.
The day that Mochi Mayhem was made publicly accessible is the same day as the annual Japanese tradition of mochitsuki (Japanese: 餅つき).[2]
Kieran apparently provides at least four tickets in total to the player for the trip, but the player would have no need for a ticket themselves as they can already access free travel to Kitakami, and in fact need to be present in Kitakami for the story to begin in the first place. Moreover, Kieran and Carmine were supposedly in Kitakami at the time, and despite the excuse of Kieran not owning a phone, Carmine could easily call the player to verify where they were and inquire about hanging out with Kieran and providing tickets for the player’s friends (Kieran notes Carmine was affected by the mochi sometime after he sent his letter).
If the player chose Sprigatito:
