Through thick and thin, Ash and Pikachu have battled their way to the top, becoming best friends along the way. But, they weren't always best friends, in fact, there was quite a bit of time before these two characters even met each other. In Pokémon Journeys: The Series' premiere episode, it explores the lives of both Ash and Pikachu prior and right up to their first meeting.
This episode has two main storylines, one following future trainers Goh, Chloe and Ash, and another following Ash's Pikachu as a Pichu.
In the episode, Pichu lives in a forest, surrounded by fellow wild Pokémon -- but it's still quite lonely. The Pokémon can't control its electricity very well yet, and doesn't belong to any kind of group, either. Pichu watches other Pokémon foster close-knit relationships with each other and grows sad, until one day it falls off a cliff into the paws of a Kangaskhan.
Kangaskhan, literally the Parent Pokémon, immediately takes Pichu in as an adopted son. Here, Pichu becomes like a brother to the little Kangaskhan living in its mother's pouch. They play together and nap together and share berries -- it looks like Pichu has finally found a family of its own.
Being a baby Pokémon, living alone in the wilderness would have been difficult for Pichu, so it's sweet to see Pichu finally find a safe place to call home. The episode is filled with many loving brotherly and motherly moments between Pichu and the Kangaskhan, with the mother even patting it to sleep in its pouch when restless. There's clearly a strong bond between them, making it no wonder that the Kangaskhan treats Pichu as one of its own.
As time passes, Pichu and the little Kangaskhan get bigger. Both these Pokémon stay in the mother Kangaskhan's pouch while traveling, but as they get bigger, their size puts more stress on her body. Like a stubborn mother though, the Kangaskhan insists Pichu travels inside its pouch despite the extra pain it causes her.
Once they settle down in a cave, Pichu decides, teary-eyed, it's time to leave its family of Kangaskhan behind. As it sneaks off from the Kangaskhan at night, Pichu evolves into Pikachu.
It's important to note, Pichu evolves into Pikachu once it feels a high amount of happiness after being treated well. Perhaps Pichu would have never evolved into Pikachu had it never met this Kangaskhan family. Prior to meeting Kangaskhan, Pichu lived alone, with no friends to speak of. Once it received a warm, loving family though, Pichu was finally ready to evolve and live more independently.
Meanwhile, though it's unclear how old he is at the start of this episode, Ash is still the same excitable, Pokémon-loving kid he's always been. He begs his mom to let him go to a Pokémon camp with Professor Oak, but unfortunately sleeps through his alarm.
Thus, the Pokémon camp goes on without Ash and we instead follow Goh and Chloe. Here the show establishes Goh's as a smug, know-it-all Pokémon fanatic, while also introducing his goal of catching a never-before-seen Pokémon. Though not as emotionally resonant as the Pichu storyline, this part establishes Ash's future sidekick and his fascination with the Pokémon Mew, which he spots during the camp.
Following the Pokémon camp and Pichu's evolution, the episode skips ahead many years later to the events seen at the beginning of the original series. The episode homages Ash's dash to Professor Oak's lab and the moment Oak gives Pikachu to Ash. But this time it shows their first meeting via Pikachu's point-of-view, as Pikachu meets Ash's excitement over how cute it is with a powerful zap.
With the context of Pikachu's backstory, its attitude seems a bit off. As a Pichu, the Pokémon was trusting of others, never showing the same level of sass it gives Ash at the start of their adventure together. It could be that, once Pichu evolved, it became far more independent, actively choosing not to rely on other Pokémon or people. Thus as a Pikachu, it takes the Pokémon far longer to trust and love those around it.
Regardless, Pokémon Journeys' prequel episode provides a wonderful backstory in Pikachu's life as a Pichu, shedding light as to why and how Pichu evolved into the Pikachu we know and love today.