The Secret of Kangaskhan (Japanese: VS ガルーラVS Garura) is the third chapter of the Pokémon Adventures manga.
Plot
Spoiler warning: this article may contain major plot or ending details.
Professor Oak suggests that Red should start his journey by heading north to Viridian Forest, where he can find Pokémon he has never encountered before. He also alludes to them not being the only thing that can be found there, but before explaining further, cuts himself off by laughing and walks away, leaving Red confused.
As Red makes his way to Viridian Forest, he realizes that because he was in such a hurry, he did not bring many Poké Balls. He then muses that as long as he has Poli and Saur, they're bound to catch every Pokémon. Meanwhile, Blue's Charmander defeats a wild Pokémon hiding in a bush. Much to Blue's disappointment, it turns out to be a Venomoth, which he already owns.
In the same part of the forest, Red is looking for a Caterpie when he sees movement in a bush and sends Poli to attack. However, he is attacked first by Blue's Charmander, who believed Poliwhirl was a wild Pokémon. Red becomes angry and tries to attack him, but Blue catches his fist. Red recognizes the boy as the one that found the Phantom Pokémon, Mew. Suddenly, they hear a loud noise, and Blue states that he found what he was looking for. A Kangaskhan shows up; Blue doesn't lose any time and attacks her with Charmander. Blue takes out his Pokédex, wanting to add Kangaskhan's data. When Red shows his Pokédex, they realize that they are rivals with the same mission. Red also learns that Blue is the grandson of the famous Professor Oak.
The fire causes a lot of damage to the Kangaskhan, and Blue tries to capture her a couple of times with Poké Balls but she repels them; however, she doesn't attack. Red perceives that something is wrong and has Poli stop Charmander's attack, and then runs over to the wild Pokémon and looks at her pouch. Her baby was poisoned, so Red uses an Antidote to cure it. The mother Kangaskhan thanks him and leaves. Blue is mad at Red's intervention, but Red still introduces himself to him, getting to learn Blue's name in response as he storms off. Red taunts him as he leaves, saying he'll never lose to him. They both leave in separate directions, both on the path to becoming the greatest Pokémon trainer of all time.
