We're pretty sure this is a common RPG trope: a mysterious and forbidden location, found just outside the hero's doorstep. If you don't end up encountering a Legendary Pokémon in here, we'll eat our Hatterene. Anyway, we're here today because a silly Wooloo broke the "forbidden seal" (OK, it was just a pretty old-looking fence) and wandered inside.
From the entrance, begin heading left. As you step into the tall grass just ahead, a wild Pokémon will ambush you. But don't fret, because you've got your starter Pokémon to protect you! The wild Pokémon in question is a low Level Skwovet, a new squirrel-like Pokémon. It's a plain Normal-type so you can just spam your Grass/Fire/Water move and utilise your Same Type Attack Bonus (STAB).
Same Type Attack Bonus
This is common knowledge for veterans, but if a Pokémon uses a move that's the same type as them, that move will deal more damage than usual. More precisely, it deals 50% more damage. For example, when a Water-type Pokémon uses Water Gun. We call this "Same Type Attack Bonus", or "STAB" for short. You'll probably see many references to STAB in this guide, so it's important we explain what it means early on.
Pokémon typically learn moves that are the same type as them, but they can also learn moves of a different type. As a rule of thumb, you always want your Pokémon to have at least one move for each of their types, so you can take advantage of STAB. That's not to say other moves are useless though; even though they don't have STAB, they can be helpful for inflicting super-effective hits.
Once you've dealt with the rabid Skwovet, continue left. In fact, you'll be going left for a pretty long distance. After you cross a small bridge, there will be another patch of tall grass. When you're about halfway through the grass, another wild Pokémon will jump at you. This time it's a Rookidee, but again it's very low Level. It's a Flying-type, so avoid using Grookey's Branch Poke as Grass is bad against Flying.
Further along, Hop will run up from behind to catch up with you. Carry on left into the third patch of tall grass. Surprise, surprise, there's another wild Pokémon hiding inside here! It's another Skwovet, so you should know what to do. When you reach the corner, start heading north. Again, Hop will come chasing your tail, but then he'll dash on ahead. Follow him into the foggy distance.
The forest trail will slowly get more foggy, until you reach the point where it's impossible to see ahead. By now, you should've caught up with Hop. As the two of you look around bewildered, a mysterious Pokémon emerges from the fog. Hey, that's got to be a Legendary Pokémon (and it looks like our Hatterene is safe)!
Wild Pokémon: ???
Ooh, how mysterious. The game doesn't tell you the Pokémon's name nor their Level. Not only that, but none of your moves will affect the Pokémon. It's like... it's a ghost... Try whatever you want. After three turns, the battle will end as abruptly as it started.
After the "battle" ends, you and Hop will both pass out. When you come to, the fog will have disappeared and Leon comes rushing in to check you're both OK. Thankfully, the Wooloo that stumbled into the forest is safe as well. So technically the mission was a success? Either way, you'll be taken outside the forest, near your house. Time to go see your Mum as you originally planned.
Go up the steps and back inside your house. Then head right, into the kitchen, where your Mum is patiently waiting. Your Mum is surprisingly nonchalant about letting a 10 year old or so kid wander around the country all on their own (and also surprisingly generous with pocket money...). Before she changes her mind, leave your house and make your way back to Route 1, where Hop is.
Who doesn't like prezzies? If you check out the "Mystery Gift" section from the main menu, you can receive gifts, get the latest news regarding the Wild Area or utilise the Poké Ball Plus (y'know, the special controller that they made for Let's Go Pikachu/Eevee). On this occasion, we want to talk about the gift-side of things.
Choose "Get a Mystery Gift" and you'll have the option to receive a gift locally, from the Internet, by using a code/password or via the Battle Stadium. At the time of writing, until 15th January 2020, you can receive a special Meowth that can Gigantamax by choosing "Get via Internet". As for the rest, try checking the official and fan websites.