While visiting a big city with Ash, Pikachu meets the Pichu Brothers—Pichu Little, the rambunctious younger half, and Pichu Big, the reserved older half. Pikachu does its best to keep Pichu Little from falling off a building, but it ends up falling off itself! The adventures continue as Pikachu and the Pichu Brothers roll off the top of a bus, fall into a river, and then fly into the air and onto the back of a Houndour! With the angry Houndour in hot pursuit, the trio makes its way to a pile of tires that serves as a “secret fort.” All the hustle and bustle, though, sends the fort tumbling down. With the help of some Pokémon, the group works together to rebuild the fort—but Pikachu remembers that it has to get back to Ash. The Pichu Brothers whisk Pikachu away and head for the building where Ash is waiting, but will they manage to get there in time?
Ash and his friends visit the Johto metropolis of Big Town complete with skyscrapers and red double decker buses. Going up in an elevator, Pikachu takes the opportunity to enjoy the view and hones in on a pair of wandering Pichu – the Pichu Brothers, who are running along a building. One of the Pichu pulls at face a Pikachu, to which Pikachu does the same back. But when Pikachu looks back the Pichu are nowhere to be seen.
The group soon reach the top floor of the building, before calling out all of their Pokémon to enjoy some relaxation time. Ash and his friends tell their Pokémon they'll be busy preparing something, though will be back by 6 o'clock and points to a nearby clock tower. The Water-type Pokémon instantly take a dip in the fountain, while Zubat spreads its wings and the other run around enjoying the open space.
Pikachu climbs up a tree and then notices the same pair of Pichu again. One of the Pichu pulls its face and teases Pikachu yet again. It then starts walking onto a narrow beam and is about to fall off. Pikachu rushes into action, taking position on a precarious flagpole and calls out just in time. Pichu manages to regain its balance and returns to the safety of the building. While Pikachu manages to prevent one from falling off a building, he ends up being attacked by a flock of Murkrow for perching on a flagpole. Pikachu ends up hanging above the city block on a rope, which a Murkrow loosens, sending poor Pikachu flying through the air, air walking across a horde of Hoppip and lands on a window cleaner's bench, sending the window cleaner, Meowth, slamming into a billboard. Pikachu befriends the Pichu Brothers, who he believes are helping him return to his friends, but they instead have other ideas and take him on an adventure across the city.
The trio gets into some trouble which includes rolling off the top of a bus, into a river, where they are nearly run down by a ferry, and are pursued across the city by an angry Houndour. They escape Houndour and find the Pichu Brother's playhouse, which appears to have been handmade by the brothers and their large number of friends from tires and scrap material. Pikachu enjoys his time with the brothers and their friends, dancing and playing around. The clock tower in the distance chimes five o'clock, leading everyone to start leaving for the day. Pikachu realizes that Ash mentioned that he and his friends would return for their Pokémon by six o'clock, and also tries to leave. Though Houndour appears and chases the trio, seemingly nearly toppling over the playhouse. Houndour is buried by some tires from the collapse but Pikachu and the Pichu Brothers save it. Houndour is grateful for their help, but the snap of one of the ropes alerts them to the fact that the playhouse is at risk of completely collapsing.
The playhouse leans dangerously, so Houndour arches its back to stop it from collapsing while Pikachu grabs a hold of one the ropes. The Pichu Brothers let out a loud cry, and soon their friends arrive to lend their strengths. Some grab the ropes and others form a chain and start passing tires along. Houndour and the others hold their positions while the Bellsprout and Spinarak use their vines and sticky threads, respectively, to add more pulling power. The last supporting rope finally breaks, making the playhouse lean dangerously. Houndour instinctively rushes in with a tire around its neck, which manages to stop the playhouse toppling completely. Everyone else changes tact and focus their attention on building up the tire foundations while a Magby uses its flames to weld sections together. Thanks to everyone's efforts, Pikachu, the Pichu Brothers and the brothers' friends straighten and rebuild the playhouse.
However, time is nearly up for Pikachu, so he and the brothers hitch a ride in a tire and race back to the building where Pikachu must meet Ash. They hit Meowth on the way, who goes flying back into his window cleaner bench. Pikachu returns to the building and bids farewell to the Pichu Brothers.
Ash, Misty and Brock arrive and lead the Pokémon into a room full of food, a tiered cake and party decorations. Ash tells Pikachu it has been a full year since they met in Pallet Town, and this party is to celebrate that. Overjoyed at this celebration, Pikachu leaps into Ash's arms for a big hug. The Pokémon start eating, and Ash allows Pikachu to wear his hat for the night. The narrator concludes the short by noting that Ash and the Pichu Brothers have something in common: neither will ever forget the day they first met Pikachu.