A Chip Off the Old Brock (Japanese: ミズゴロウとモココ！恋の特効薬！？Mizugorou and Mokoko! Wonder Drug of Love!?) is the 148th episode of the Advanced Generation series, and the 422nd episode of the Pokémon anime. It was first broadcast in Japan on October 20, 2005 and in the United States on September 16, 2006.
Brock's Mudkip evolves into Marshtomp while training with Grovyle, but when it shows off its Mud Shot attack, it accidentally hit s a nearby Flaaffy! Luckily, Flaaffy's owner, Mariah, works at a pharmacy that makes Pokémon medicine. Soon Flaaffy is well, but Marshtomp has developed a crush on it! Mariah and her friend McCauley are helping the pharmacy's doctor, using Electric Pokémon to help make medicine. It's not easy, and McCauley is really starting to doubt his abilities.
A disguised Team Rocket tries to clean out the pharmacy using Meowth in a Snorlax suit, but McCauley and Brock drive them off. Team Rocket doesn't give up: later that night, they return with a rocket-powered balloon and steal the entire building while Mariah and Flaaffy are still inside! McCauley and his Ampharos leap into the building as it's carried off. Back on the ground, Ash and his friends try to stop Team Rocket, but Pikachu and the doctor's Mareep are hurt when Marshtomp's attack is deflected by Team Rocket's latest device.
Up in the flyaway pharmacy, McCauley and Mariah mix up a Pokémon medicine, then toss it to the doctor. The hurt Pokémon are restored to full health, and the Electric Pokémon pool their power to defeat Team Rocket. McCauley now has confidence in himself, but poor Brock and Marshtomp realize that romance with Mariah and Flaaffy will never pass! Yes, everything's definitely back to normal as our friends head towards the next stop on the Battle Frontier!
Brock offers to help, but Flaaffy’s Trainer, Mariah doesn’t respond. Instead Mariah runs to her Grandma Katie's drugstore for treatment, to which Ash and his friends follow after her. After a quick sip of medicine, Flaaffy heals in an instant. Ash and Brock immediately apologize and introduce themselves. Then, Brock and Marshtomp hold Mariah and Flaaffy's hands and show attraction. However, Max and May jump in and pull the lovesick twosome away. Suddenly McCauley and his Ampharos enter the store to show Katie his energy elixir. After a quick examination, she notes that McCauley didn't mix the Cheri Berry well, so she can't sell it. McCauley is disappointed by his efforts, admitting that he may never improve. Mariah tells him to focus on the positives and that he can always try again. Outside, Team Rocket spy on the drugstore and plot to steal and on-sell the medicines.
Katie orders McCauley to mind the drugstore while she makes medicine. Brock also asks to see the medicine-making process, so he May, Max go with Katie and Mariah, while Ash and Pikachu train for their next Frontier battle. In her storeroom, Katie mixes some ingredients into a watery paste and Brock takes notes as they watch Katie make medicine. When all the ingredients are mixed Katie puts it into a bottle and uses her Mareep's Thunder attack to purify the medicine. Katie admits the amount of electricity required is still a rather experimental process.
Back at the drugstore, Jessie and Jamesdisguise themselves as an elderly couple with an ill Snorlax; none other than Wobbuffet and Meowth inside a costume. Jessie and James begin clearing the shelves, throwing all of the medicine directly into the Snorlax costume. McCauley is alarmed by the amount of stock the duo are taking, when Jessie suddenly throws him and his handful of pharmaceuticals into the costume’s mouth. McCauley immediately sees Meowth and Wobbuffet inside and realizes that it isn't a real Snorlax. Team Rocket promptly run away with the medicine, but Ash sees them and commands Grovyle to use Bullet Seed to trip them up. Team Rocket's disguises fall off, revealing their true identities. They don't give the medicine to McCauley, so Brock tells Marshtomp to use Mud Shot, and Team Rocket falls into a boat and are carried down a waterfall. Mariah rushes out to see if everything is alright, and McCauley admits the medicine has been all recovered. However, Mariah blushes as she adds that she was really asking about him.
During the night, Brock combs Marshtomp and tells him that together with Mariah and Flaaffy they will be good couples. Then, Ash notices an incredible amount of electricity coming from inside the drugstore, so the group decide to investigate. Inside is Ampharos and McCauley practicing to make medicine but the bottle cracks open from the overcharge. May tells him that it is too late to practice, but McCauley says that he wants to be as good as Katie. He explains that he and Mariah have been watching Katie make medicine since they were both children. As McCauley admits he’ll never come good, Katie enters the room and assures him that he needs to improve his confidence to succeed. McCauley commits to be more confident in himself.
Unexpectedly the whole building begins to shake as Team Rocket lifts the drug store off the ground. Everyone escapes in time, but they realize that Mariah and Flaaffy are still trapped inside the store. While Team Rocket recites their motto, McCauley climbs up another house with Ampharos and they both jump inside the open store door. In realization of his attempt, Jessie commands Meowth to shake McCauley off. Subsequently all of the stocked medicines smash and break on the ground. To stop Team Rocket making a getaway, Brock orders a Mud Shot from his Marshtomp. Meowth retaliates by deploying a protective dish that sends the attack right back at them, injuring both Pikachu and Mareep. Katie quickly asks for some Electric Pokémon medicine to heal the Pokémon, but Mariah and McCauley inform her that the medicine has been smashed. Katie is angered at this, and Team Rocket only respond by laughing at her nonsense bickering. James's Mime Jr. jumps up to the balloon's controls and starts playing with them happily, making the balloon and the store rock even more. McCauley spots two remaining medicines that weren't destroyed, and he decides they must take action quickly even without the magic Sunkern dew. Ampharos, with all its electric might appears to be struggling mixing the medicines, so Flaaffy joins in, but before they are done Team Rocket fixes their rocket boosters. Instantly, McCauley throws the newly made medicine down to Katie. She promptly heals Pikachu and Mareep and they both deploy a ferocious double Thunder. Meowth activates the reflective dish once again, but the pair dodge the reflected attacks in time. Inside the floating store, Ampharos and Flaaffy use the same attack to successfully destroy the balloon engines and make the ship crash down. On ground, Mariah, McCauley, Ampharos, and Flaaffy escape the store to leave Ash and his friends to fight Seviper and Cacnea. James has Cacnea attack with Needle Arm and Seviper uses Poison Tail while Marshtomp uses Water Gun to crash both of the Pokémon into Team Rocket. Ampharos and Flaaffy use Thunder to immobilize Team Rocket while Pikachu and Mareep also use Thunder to send Team Rocket blasting off again.
Afterwards, Mariah realizes that McCauley's confidence was the reason they created the medicine and defeated Team Rocket, and Katie, seeing this, gives the store to them. Brock and Marshtomp are let down and left heartbroken at this. The following day, May thanks Katie for the medicine supplies as she and her friends continue their journey towards the Battle Arena.