Jessie states that she and Mimikyu have mastered their Z-Move. In celebration, Team Rocket performs the Z-Move, but just as the Z-Power is passed onto Mimikyu, a call from Matori comes through. Matori asks about the trio's progress, and Jessie proudly displays her the recently obtained Z-Ring. Jessie then reveals that they are intent on catching Pikachu for the Boss. Matori notes that knowing a Z-Move will help them to catch rare Pokémon, and commends Jessie's enthusiasm before signing out. Jessie is left infuriated by Matori's disruptive and frequent check-ins. Meanwhile, Bewear continues to stockpile Berries, pausing for a moment to listen to the decoy Team Rocket's dialogue.
Outside Acerola's home, Ash prepares Lycanroc and Pikachu for some training. The technique surprises Acerola, though Rotom assures her that this is normal for Ash. Pikachu begins with a Quick Attack, though Lycanroc responds with Accelerock. Pikachu lands an Iron Tail and follows it up with an Electro Ball. As Pikachu launches its attack, he slips in a mud puddle and the attack instead sprays Lycanroc's face with mud, but fortunately, it does not erupt into anger. As Ash addresses Pikachu, he notes that Pikachu's Electro Ball went amiss and Rotom adds that it appeared to split apart for some unknown reason. Ash declares the practice match a success and looks forward to challenging Nanu.
At the Po Town Police Station, Nanu is busy playing around with his kendama when he is startled by Team Rocket's arrival. They ask for his assistance to find Ash, claiming that they wish to have a battle against Pikachu. Nanu blindly points to a map, and explains that Team Rocket can find Ash at the abandoned Thrifty Megamart. As Team Rocket makes their way there, Ash soon arrives at the police station, demanding a trial against him. Nanu agrees to a Grand Trial battle against Ash on the condition that he defeat some Trainers and their Mimikyu at the abandoned Thrifty Megamart first. He also elects Acerola to judge the match. Ash begins making his way over to the abandoned site, while Nanu smirks at his cunning evasion tactic and returns to relaxing.
At the abandoned Thrifty Megamart, Team Rocket awaits their opponent. Meowth becomes tired of waiting and decides to look behind a stall shelf. He screams, directing Jessie and James' attention to a set of glowing eyes. They are all terrified, but quickly recover when they learn it is just Jessie's Mimikyu. Suddenly, a shopping cart nearly falls on Team Rocket, forcing them to briefly flee. Elsewhere, Poipole pushes several of the shopping carts towards Pikachu, who evades them. Ash tells Poipole to stop fooling around, but Rotom reassures him that Poipole enjoys practical jokes. His attention soon turns to Team Rocket, who perform their motto. Ash quickly realizes that his trial opponents are Team Rocket and their Mimikyu. Acerola announces that she will be the judge, and Team Rocket bow in respect of her authority. Left with no other choice, Ash officially accepts the challenge.
Bewear finally goes into Team Rocket's base in its den. It looks at the dummies, who soon begin to repeat their lines and fail. Realizing the real Team Rocket trio is away, Bewear screams. It jumps out of its base, leaving a path of destruction in its wake, and goes out to find them.
Meanwhile, at the Pokémon School, Professor Kukui is in the midst of teaching a class based on Mimikyu and asks what should his students be aware of when battling it. Lillie states that Mimikyu is immune to Dragon, Normal, and Fighting-type moves. Kiawe is then reminded of Team Rocket's Mimikyu while the other students wish to see what is under its Pikachu-shaped cloth. Kukui explains that Mimikyu are more common on Ula'ula Island than anywhere else in Alola. At that moment, Kiawe and Mallow wonder how Ash's trial is going.
Back on Ula'ula Island, Pikachu lands a successful Iron Tail, but it only triggers Mimikyu's Disguise Ability and does little damage to it. Mimikyu retaliates with Play Rough, followed by Shadow Claw. Ash has Pikachu use Electro Ball to avoid close combat, one of Mimikyu's strength. However the attack is returned to him as Mimikyu uses Wood Hammer. Ash tries another Electro Ball; this time, the attack splits, like what happened earlier that day, and hits Team Rocket instead. Pikachu dodges Mimikyu's approach with Quick Attack, but its Thunderbolt is interrupted by a Shadow Claw. Just as Jessie is about to perform a Z-Move with Mimikyu, Meowth is distracted, thinking that James touched him. James denies it, but he suddenly feels something touch him as well. At that moment, Bewear arrives and prepares to take Team Rocket away, despite their pleas. However, Mimikyu stares darkly at Bewear, prompting it to free Team Rocket and allow the battle to continue.
Mimikyu turns its attention to Pikachu and unleashes several Shadow Claw attacks, which he dodges and counters with Iron Tail. Mimikyu's Wood Hammer clashes with Pikachu's Iron Tail, before Mimikyu resorts to knocking down several shop stands. The two foes continue to exchange blows amongst the obstacle course, running along shelve tops and in-between the aisles. Pikachu dodges another Shadow Claw, and responds with Electro Ball. Mimikyu deflects the attack, which instead nearly hits Team Rocket. Spotting Pikachu's exhaustion, James, Meowth, and Wobbuffet launch into their Z-Move pose with Jessie, who twists the Mimikium Z on the Z-Ring. Pikachu manages to stand up, but it is too late as Mimikyu performs Let's Snuggle Forever. Mimikyu drapes its costume over Pikachu and beats its opponent, leaving Team Rocket shocked by the amount of violence displayed. Jessie tears up, believing her fortunes have finally changed and anticipates the Boss's praise.
However, as Mimikyu jumps off, Pikachu is revealed to have unknowingly protected himself in an Electroweb. Proud of the new move, Ash has Pikachu trap Mimikyu with another Electroweb. Ash declares that he will respond with his own Z-Move, and his Z-Crystal again changes into a Pikashunium Z. Ash places his hat on Pikachu and the two perform 10,000,000 Volt Thunderbolt. The multicolored thunder beams hit their mark, blasting a large hole in the Thrifty Megamart's ceiling and leaving Team Rocket burned. As Mimikyu faints, Acerola declares Ash and Pikachu the trial winners. Ash happily embraces Pikachu, thanking him for his efforts, while Jessie demands Mimikyu to continue going, only to be stared down by Bewear, who gently picks up the unconscious Pokémon. Bewear then drops Team Rocket into a shopping cart and carefully places Mimikyu on top of the heap. Using the collapsed ceiling as a ramp, it quickly wheels the trolley up and flies out of the area.
Nanu arrives on the scene and is irritated by Ash's victory, but concedes and decides to keep his promise to Ash. Ash is overjoyed by this and thanks Nanu.