Ash Ketchum is finally 10 years old, old enough to become a Pokémon Trainer. He wakes up late on the day he is supposed to receive his first Pokémon at Professor Oak's lab, however, and arrives to discover all the Pokémon are gone. Ash has no choice but to take a naughty Pikachu. Pikachu won't listen or go inside its Poké Ball, so Ash must drag it along with a rope. After many failed attempts at capturing some Pokémon, Ash throws a rock and hits a Spearow. The Spearow gets angry and the entire flock starts chasing Ash and Pikachu. They run away from the Spearow, but when they cannot go any farther, Ash uses his body to shield Pikachu from the attacking Pokémon. Pikachu realizes how much Ash cares for him and uses a powerful electric attack to drive the Spearow away. Note: The previously unknown Pokémon Ho-Oh is seen towards the end of the episode.
Finally arriving in Viridian City, Ash is cornered by Officer Jenny who asks why he is carrying Pikachu instead of having it in a Pokéball. After explaining, Jenny takes Ash to the Pokémon Center where Nurse Joy begins treatment on the injured Pikachu. Before Pikachu’s treatment finishes, however, Ash is interrupted by Misty, who vows to stick with Ash until he pays her back for destroying her bike in the previous episode. The Center is then attacked by two members of Team Rocket (Jesse and James). Ash's Pikachu and the Center's many Pikachu defeat Team Rocket by shocking them.
Ash, Pikachu and Misty have entered the Viridian Forest. Ash captures his first Pokémon, a Caterpie, and later, a Pidgeotto. Misty, however, is scared of bugs and screams every time Caterpie comes near her. Meanwhile, Team Rocket is following them, ready for a fresh attempt at capturing Pikachu. Can he, Misty, and his Pokémon fend off Team Rocket again?
While wandering through Viridian Forest, a Samurai challenges Ash to a battle, and it turns out to be Metapod vs. Metapod. After a short time, a swarm of Beedrill attacks, and Ash's Metapod is taken by them. Ash is able to rescue his Metapod and it evolves into a beautiful Butterfree.
Ash and Misty finally reach Pewter City and meet Flint, a mysterious man who sells rocks. Ash is ready to challenge the Pewter City Gym Leader, Brock, for his first Badge, but loses the battle. Flint takes Ash and Pikachu to Brock's home and Ash learns that Brock cares for his many younger brothers and sisters since their dad left them. He then brings Pikachu and Ash to a watermill to make Pikachu stronger. Ash battles Brock again, this time coming closer to winning. A fire breaks out in the gym, however, and the sprinklers go off, weakening Onix. Ash wants to defeat Onix but is held back by the thought of Brock's siblings (when he opens his eyes he finds he was literally held back by them). Ash leaves the Gym but Brock catches up to him and gives Ash the Boulder Badge. Flint turns out to be Brock's father and takes over the Gym and the responsibility of his children for Brock when he decides to join Ash on his journey.
Ash, Misty, Brock, and Pikachu arrive at Mt. Moon to find Team Rocket interfering with the local wildlife in hopes of finding the rare Moon Stone. With the help of Seymour the Scientist, the gang saves the Moon Stone and the Clefairies that guard it. Also, Brock catches a Zubat.
In his quest to collect Trainer Badges, Ash heads for Cerulean City, intending to confront the Cerulean Gym Leader. For some reason, Misty expresses a desire to avoid Cerulean City and disappears along the way. Upon entering the gym, Ash and Pikachu are entranced by a synchronized swimming show being performed by three beautiful sisters. Could this truly be the Cerulean Gym? And what could be the role of the three beautiful swimming sisters? More importantly, will Ash be able to defeat the strangely absent Gym Leader to earn his next badge?
After receiving his second badge, Ash sets off triumphantly for Vermilion City. Along the way, they hear of a trainer named A.J. - a fierce Pokémon trainer who has never lost a match. Full of self-confidence from his own record of ten victories in a row, Ash heads directly for the gym to find his new opponent. When they arrive, a huge bulletin is in front of the gym with a neon sign declaring “98 Consecutive Victories”, which appeals to Ash’s over-confidence. A.J. is on the brink of achieving his goal of 100 straight victories, after which he will embark on a journey to gather badges. Will victory fall on the side of Ash and his 10 straight victories or with A.J. and his string of 98? In the process, will Ash learn some important lessons about the bonds between Pokémon and trainer?
Our three heroes have lost their way in the depths of a dense wooden forest, shrouded in thick, white mist. Momentarily giving up, they decide to break for a brief rest. As Ash wanders deeper into the wood, searching for firewood, he happens across a young boy being bullied by five classmates gathered around him. Eavesdropping on their conversation, Ash learns that the boys are attending a preparatory school for Pokémon trainers. Graduates from the school are automatically qualified to participate in the Pokémon League without travelling around and collecting any badges. Ash, Misty, and Brock head to the school and don’t agree with some of their ideas of training, something their "leader" doesn't take well.
Ash and friends are still on their way to Vermilion City and still lost. They stop by a pond for a break when they see an Oddish taking a drink. Ash and Misty try to catch it but they are stopped by a mysterious Bulbasaur who rescues the Oddish. They continue on their journey and keep getting caught in traps, until the Bulbasaur's owner, Melanie, finds them, saves them and brings them back to her cabin. It turns out that she helps injured, sick, and abandoned Pokémon, and they decide to help. Meanwhile, Team Rocket decides to steal all the Pokémon, while Brock wonders if he should stay with Melanie. In the end, Melanie gives Bulbasaur to Ash.
While on the road to Vermilion City, Ash, Misty, Brock, and Pikachu come across a Charmander sitting on a rock. Ash tries to catch it, but it turns out it has a trainer and can't be captured, so they head to the Pokémon Center. At the center, a hotshot trainer named Damian is bragging about how he left it, promising to come back for it when he means to abandon it. After hearing this, they head back out to get the poor Pokémon as a rainstorm threatens its very life. After a battle with Team Rocket, where Damian witnesses Charmander defeating them, he tries to get Charmander to come back to him, but the Pokémon's loyalty now belongs to Ash. Ash captures Charmander, gaining a new friend and Pokémon.
Ash, Misty, Brock, and Pikachu become the victims of pranks played by a gang of the Squirtle Squad. They end up falling down a hole, and are even eventually captured by them. It turns out that they are working together with Team Rocket. When Pikachu is severely injured, Squirtle let Ash out to get medicine from town, but must return soon or his friends will be trapped by the Squirtle Squad forever. In the end, the Squirtle Squad save a town from a fire started by Team Rocket and the lead Squirtle joins Ash on his further adventures.
When Krabby that Ash catches disappears, he goes to a nearby lighthouse to see if it reached Professor Oak's lab. The keeper of the lighthouse is Bill, a Pokémon Watcher who is searching for a mysterious Pokémon that sometimes appears by his lighthouse. Just as this mysterious Pokémon answers Bill's call and approaches the lighthouse, Team Rocket appears and tries to capture it. Injured, the Pokémon loses its trust of Bill and vanishes forever, though Bill intends to keep an eye out in case it ever appears again.
Ash finally arrives in Vermilion City and is ready to prove that he can defeat its Gym Leader. However, his opponent has a very tough Raichu that keeps the Pokémon Center very busy...
Ash 'wins' a ticket to take a trip on a luxury cruise ship, the St. Anne. However, this is a scheme devised by Giovanni, who plans to steal the Pokémon of all the trainers onboard.
The St. Anne flips over and sinks to the bottom of the sea after a perilous storm. All the passengers manage to escape, except for Ash, Misty, Brock, and Team Rocket, who are trapped on board. After regaining consciousness, Ash, Misty, Brock, Pikachu, and Team Rocket discover their predicament and have to work together to escape.
After being hit by a Dragon Rage attack from James' newly evolved Gyarados while at sea, the humans are separated from their Pokémon on an island containing many giant Pokémon. They manage to escape leaving the island in ruins, which happens to be run by none other than the wicked Team Rocket boss Giovanni himself.
Stuck in Porta Vista because they missed the boat, Ash and friends sit around waiting for the next boat and witness a great explosion. They later discover that it is a group of Tentacool that is attacking the city since construction is destroying their reef. Ash and his friends manage to appease the giant Tentacruel and continue on their journey
The boat that Ash and his friends are on docks at a small town celebrating Obon. Brock and James both fall for a beautiful girl standing by the sea who disappears. Ash, Misty, and Team Rocket learn that she is the ghost of a maiden who waited for her lover to return from a war and eventually turned into stone. While trying to protect Brock and James with some stickers bought from an old woman, the maiden reveals herself as the old woman, who in turn reveals herself to be Gastly, a ghost Pokémon. Ash tries to fight Gastly to release James and Brock from its spell but fails, until the sun rises and Gastly is forced to disappear. It is revealed that the ghost of the maiden actually exists and is a friend of Gastly, and is still waiting for the one she loves to return.
Team Rocket encounters the ghost Pokémon in a haunted tower while setting a trap for Ash and his friends, and end up getting frightened away. Ash and Pikachu enter alone since his friends are afraid to go in. Inside, he and Pikachu are the victims of pranks by a Gastly, Haunter, and Gengar looking for playmates. After being crushed under a falling chandelier, Haunter turns Ash and Pikachu into ghosts where they have some fun, though Ash asks them to send him back after seeing Misty and Brock cry over his lifeless body. Ash and Pikachu are brought back to life and Haunter decides to follow Ash as his new Pokémon.
When Ash reaches the Celadon Gym, he is kicked out. He soon discovers that it is only for people who love perfume. So, with the help of Team Rocket, Ash sneaks into the Gym disguised as a girl. In the Gym, his disguise is ruined by Misty, Brock, and Pikachu. During the battle, Team Rocket interrupts and sets the Gym on fire. After rescuing Gloom, Ash is presented with the Rainbow Badge.
When Ash and his friends see a group of Growlithe training against a Policeman posing as a robber, Ash decides for him and Pikachu to do the same training to become the best they can be. However, when Team Rocket comes to steal them, Officer Jenny, Ash and the gang's voice gets changed, causing the Growlithe to think that Team Rocket are the Officers as they use their ears to follow command.
