During lunch, Lana tells her classmates about how Harper and Sarah got lost but were assisted by a mysterious Pokémon that then gifted them with a red berry each. With Lana's description, Rotom searches its system and identifies the berries as Haban Berries. Mallow screams, as she and Lana remember "Grandpa Forest", whom Lana assumes her sisters must have met. Mallow and Lana explain that when they were small, they also got lost in the forest and a big Pokémon helped them out. Mallow admits she doesn't remember its appearance, and despite searching for it, the pair have never seen the Pokémon since. Lana hopes to thank the Pokémon, so Ash decides that the group should go search for it. Meanwhile, Team Rocket, huddled under a staircase, overhear the conversation. After fantasizing about its rarity, the trio decides to follow Ash and his classmates directly to the Pokémon.
Ash is slightly disappointed that only he, Lana, and Mallow are able to make the journey, since their other friends had commitments. When Lana becomes adamant that they will find Grandpa Forest, Mallow remembers that it was also the day she first met Lana. Years ago, Lana came with her mother to pick up a delivery from Aina's Kitchen. Mallow startled Lana as she was playing in the grass, and asked her what was she doing. Mallow then introduced herself, while Lana silently muttered out her name. Mallow grabbed Lana by the arm and pulled her into the nearby forest and to her favorite spot, a flower field that enchanted Lana. Mallow made Lana a flower bracelet, before a Cutiefly appeared. The two decided to follow it, which led them to a horde of Ribombee. After showing some interest in Lana's bracelet, Lana gifted it to Ribombee as a crown. In return, Ribombee handed them a Pollen Puff each. Mallow encouraged Lana to try the sweet and she did, much to her delight.
Team Rocket, disguised as Bug-type Pokémon, overhear the conversation and look forward to eating Pollen Puffs. James notes the delicacy is sweet and nutritious. Jessie eagerly searches for the nearest supply and comes across a Ribombee. The small Pokémon is startled and fashions a purple puff for Jessie, but as Jessie consumes it, she ends up getting stomach pains as James reads that Pollen Puffs can also be used for attacking.
Ash, Lana, and Mallow stop in front of a tree, which they say is where they met Grandpa Forest. Again reminiscing, Mallow and Lana reveal they were playing with the Ribomee before it flew away. The two followed and soon discovered a strange, large Pokémon. The two were terrified and tried to walk away from it, but before they could, the Pokémon lowered its head and roared, causing Lana and Mallow to run away in terror. Rotom promises to guess the Pokémon if they describe its characteristics. Mallow and Lana explain that the Pokémon looked old and was white and green. Rotom reveals that the Pokémon was an Abomasnow, but Lana immediately denies it, angering Rotom, who demands a better description.
Mallow and Lana continue remembering the events. As they ran off, Lana and Mallow got lost in the forest. Mallow apologized for forcing Lana to go with her into the forest, and the two later became hungry. Mallow started crying, fearing they would never be able to return home. Lana, however, noticed some Haban Berries on the floor. In hunger, Lana ate one, only to find it bitter. Regardless, she gave one to Mallow, who ate it and was also shocked by the bitterness. Mallow called Lana out for lying about its taste, but Lana was glad that she stopped Mallow's tears. Meanwhile, Mallow became glad that Lana called her by her name instead of by the nickname "Miss Family Restaurant". The two then ate the Berries, and noted they were starting to become sweeter, causing them to dub them "Bittersweet Berries". The two walked away and Mallow soon recognized the path that would take them home.
Suddenly, Lana tripped over and dropped her Berry. It slid down, so Lana pursued it, only to nearly fall down a cliff. Mallow grabbed Lana by her shirt, but the vine she was hanging onto snapped and the pair were sent falling. Fortunately, the Pokémon they had seen earlier saved them. With the new information, Rotom finds the answer and explains that the Pokémon is a Drampa, surprising Lana and Mallow; Team Rocket are also relieved to learn of its identity. Rotom tells Ash that looking in a Berry-filled location is a good start to finding Drampa. Pikachu's ears twitch as it smells the air, and it then runs off. Ash, Lana, and Mallow follow Pikachu, who leads them to a clearing surrounded by Haban Berry trees. Their Pokémon stumble upon Grandpa Forest, who is happy to see Ash and his friends.
Suddenly, Team Rocket appears, dressed in Preschooler apparel. However, Ash, Lana, and Mallow don't fall for the disguise and look in disbelief as Team Rocket try to woo Drampa, who doesn't believe it either and attempts to fly off, only to be pulled down to the ground. As Jessie and James chastise Meowth for the failed plan, but then, he reveals their backup plan, and his baby bottle transforms into a net gun. Meowth fires out a net, which ensnares Drampa. Pikachu goes to save Drampa, who becomes angry and frees itself from the net. Drampa traps Team Rocket inside a Twister, however Bewear soon appears and rescues the trio.
Afterwards, Lana and Mallow confront Drampa, who recognizes the girls instantly and hugs them. Ash is also amazed to meet Drampa and hugs it. Lana and Mallow thank Drampa for rescuing them and Lana's sisters. Ash picks some Haban Berries from the trees and shares them with everyone. Lana and Mallow eat their shares, only to again be startled by the Berries' bitterness.
Later, Lana, back at her house, watches as Harper and Sarah draw Drampa and the Haban Berries, and approves of their drawings.