• Into the Woods
If you brought your Dowsing Machine up - which I recommend at all times - then you will notice that there is a hidden item behind the fence right here at the entrance. Go around the fence and inspect the area at the end to find a Super Potion. If you didn't grab TM94 Rock Smash as recommended earlier, then you can grab it now from the very first girl along the fence. If you talk to the nurse dressed in pink near the entrance to the deeper part of Pinwheel Forest, she will battle you and then heal your Pokemon at any time afterwards. This can be very helpful if you find yourself low in health around the area and don't wish to stalk back to the city's Pokemon Center. Follow the path behind the fence to nab an Ether at the end.
Proceeding deeper into the forest, you will find a few more trainers and a lot of items. Before heading too deep, there is a Great Ball to the right by the broken railroad tracks. Through the patch of grass into the south, an antidote is found along this path. A TinyMushroom can be found past the dark grass on the left, a super potion in the bottom left, an antidote hidden in a large puddle in the south, and a few more items to the east. If you head east, you will climb up a staircase and be presented with a couple trainers. One will reference the Challenge Rock, which is the large rock located in this area. Inspect the rock with a Fighting-type Pokemon like Pignite to obtain a Star Piece. This can be done once daily. There is a hidden ether in this area. To get back to where you started, head down the staircase west of the Challenge Rock. A net ball is located along this path. Prepare to enter the depths of Pinwheel Forest.
As you venture into the heart of Pinwheel Forest, you will hear the ominous Team Plasma theme playing, letting you know that Team Plasma is indeed waiting for you along the path ahead. Burgh will talk with you, saying that there are two paths that go through Pinwheel Forest. One is the nicely paved route right up through the middle and to the exit. Burgh will be taking that one. We'll be taking the extremely complicated and winding path through tall grass that is infested with trainers instead. Joy. Already you can see a couple trainers waiting for you, Twins and a Team Plasma Grunt. As usual, the Team Plasma grunts will be using primarily Normal- and Dark-type Pokemon. If you have a fighting-type attack, it could prove very valuable throughout the forest.
This area is just as infested with Pokemon and items as it is with trainers. There is also a mysterious Moss Stone located just north when you enter the inner path of the forest. Leveling up an Eevee here will evolve it into Leafeon. That comes later in the game, however. On the opposite side - the south branch of the main path - you can find a Big Root. This increases the health recovered from health sapping moves. Have your Dousing Machine ready, as well, because right away there is a narrow grass-filled path just south of a visible Super Potion where a TinyMushroom rests.
Heading further up along the linear path, there is a hidden Pokemon Ranger that gives you Chesto Berry for defeating her. The same with a trainer up ahead, except you receive a Pecha Berry instead. There is also yet another TinyMushroom hidden in the dead stump. A Paralyz Heal and another ranger with a Pecha Berry can be found along the trail. If you head down and around the winding path you will find TM86 Grass Knot. This move does more damage to heavier opponents. We cannot access the other part of this area yet.
Heading through the hollow log, you will encounter the final Team Plasma Grunt in our little trek. After the battle, he will forfeit the Dragon Skull that was stolen from the museum. As a rather... unwelcome surprise, a man named Gorm will come forth and address the Team Plasma Grunt as a fellow subject of the King. The Grunt takes surprise in this, and addresses him back as Gorm of the Seven Sages. It seems there is more to this group than we thought! As it turns out, the hijacking of the Dragon Skull was completely unnecessary and was actually unrelated to their goal - unbeknownst to the Grunts that coordinated the effort. It is also revealed to us that Ghetsis is not, in fact, the leader of Team Plasma and the King of which Gorm spoke. Ghetsis is but another one of the mysterious Seven Sages.
We will now have a brief conversation with Lenora and Burgh. Burgh, the Gym Leader of Castelia City, invites you to his gym. Lenora will also take the Dragon Skull from you, rewarding you with a valuable Moon Stone. This can be used to evolve certain species of Pokemon, such as Munna. Heading over the the ledge to our right, there is a Miracle Seed. If you want to train, you can battle some of the trainers on this bridge. If not, head up and out of Pinwheel Forest.