To celebrate the Pokemon anime's 20th anniversary, we're recapping an episode of the series every Saturday!
Episode Title: Ash Catches a Pokemon!
Episode Synopsis: Ash quickly catches the Caterpie he encountered last episode, much to Misty's surprise and dismay. Misty has an irrational fear of Bug Pokemon, and she hurts Caterpie's feelings when it tries to befriend her. Ash sticks up for his new Pokemon, leading to yet another bicker fest between Ash and Misty. The next morning, Ash encounters a Pidgeotto and sends Caterpie out to battle it, not realizing that Flying Pokemon have a natural advantage against Bug Pokemon. With Pikachu's help, he captures Pidgeotto, but is immediately dressed down by Misty for putting Caterpie in danger. Team Rocket shows up and challenges Ash to his first ever Pokemon battle. Pikachu is blinded by poison in the eye and Pidgeotto is beaten by Koffing and Ekans double teaming it. However, Caterpie has better luck with its String Shot attack, incapacitating all of Team Rocket's Pokemon. Afterwards, Caterpie evolves into Metapod.
Characters Introduced: Pidgeotto
Locations: Viridian Forest
Ash's Moment of Idiocy: Ash sends out Caterpie against a Pokemon that eats worms. In fact, we see Pidgeotto eat a worm before fighting Caterpie
The point of "Ash Catches a Pokemon" is to introduce viewers to the concept of Pokemon battles. We see Ash battle both wild Pokemon and trained Pokemon, albeit at an unfair disadvantage. Ash is also established at being pretty bad at Pokemon battles, relying more on dumb luck than any sort of strategy to win. This becomes something of a running trend throughout Ash's first few years as a Pokemon trainer; he doesn't really come into his own until he reaches the Indigo League.
Ash's team also gets a major expansion during this episode, capturing both Caterpie and Pidgeotto. Caterpie is actually given some characterization, it's kindhearted and gentle, and desperately wants to be friends with Misty. I don't know how Misty could hate Caterpie after seeing it cry, but she barely makes an effort with the Bug Pokemon even after it saves her and Ash from Team Rocket. Caterpie also tells Pikachu (or at least, it's presumed it tells Pikachu...we don't actually know what Caterpie says) that it wants to become a Butterfree...a goal that doesn't take too long to accomplish.
While Caterpie shows heaps of personality, Pidgeotto just sort of exists in the episode. We see that Pidgeotto is a competent battler, almost pulling off a two on one victory against Team Rocket, but it doesn't show even a scrap of personality during this episode. Pidgeotto is probably the least developed of all of Ash's "permanent" Pokemon, with no personality quirks outside of an unfailing ability to obey Ash's orders.
Although it's a bit buried in the middle of Team Rocket's battle sequence, the episode also establishes that Pikachu is "special" for its species. Meowth comments that Pikachu's power exceeds its evolutionary line, which explains why it was able to level a Pokemon Center with a single blast.
As for Team Rocket, they're still presented as a very real threat to Ash and Misty...although we see a bit of humor when they prematurely celebrate beating Ash during their battle. It's interesting to see Team Rocket presented as real viable threats, instead of just trainers for Ash to quickly level up his Pokemon.
The episode ends with our first look at a Pokemon evolving. Caterpie evolves into Metapod, albeit without the spontaneous transformation that we see in other Pokemon evolutions. Instead, Caterpie covers itself in its own String Shot. This seems to be a one-time thing, as future evolutions almost always feature bright flashes and Pokemon converting into energy to change its form. Interestingly, Ash reintroduces himself to Caterpie, hinting that he knows that Pokemon don't always retain their old personalities when they evolve.