Brock was bored at the moment. The group had been travelling through this forest for what seemed like forever and the scenery was starting to blend together. Ash, Dawn and himself were travelling near Canalave, hoping to catch a boat as Ash had wanted to head back to Kanto for a little while. Brock noticed they were low on food and excused himself, under the guise of hopefully being able to find some berries or other fruit to eat.
As he walked by himself, an odd rock caught his attention, however. Bending down, Brock noticed that it bore a passing resemblence to the Soul Badge that Koga and his sister gave out to trainers, only a dark gray. Finding it interesting, he picked it up. As soon as he did that, however, the rock began to glow! Brock, shocked, began to feel lightheaded. In fact... he felt light in general. Looking down, he couldn't believe his eyes: he was floating! Surprised, he reached to grab a tree to steady himself, only to find his arm go right through it.
Realizing quickly that it must be because of the stone he found, Brock looked down and saw it was nowhere to be found. Giving up, Brock decided to float through the forest for a bit. It wasn't long before he spotted somebody not too far from where Ash and Dawn were. Getting closer, his heart stopped: it was a pretty girl. In fact, he recognized her as Gardenia, the Eterna City Gym Leader they had run into some time back. She was a little far from her Gym, though Brock figured that maybe she was on the lookout for any new Grass Pokemon.
Brock floated down in front of her, trying to get her attention though she seemed to not be able to notice him. Reaching his hand out to her, he suddenly felt himself get sucked forward. Coming to, he felt different. He felt shorter and lighter. His hair was still short but he noticed two long, auburn bangs hanging down the side. And he was wearing... a green shawl? It dawned on him: that rock had somehow turned him into a spirit, capable of possessing others and right now, that "other" was Gardenia!. Thinking, he tried exiting Gardenia's but nothing happened. He felt something in her pocket, though, and reaching in, he pulled out the stone.
It must've travelled with him somehow, he realized. Thinking, the stone began to glow and he soon found himself floating over Gardenia.
"Ugh, what happened?" she asked herself, shaking her head. "Did I black out or something?"
Brock realized that this could be quite useful indeed. As he pondered how he could finally get as close to any girl as he wanted, he heard Ash and Dawn nearby, calling out for him. Brock was left with a dilemna: take over Gardenia's body again and explain what happened, possess Dawn, waiting until he could find a better body, or he could take over Gardenia's body again, but live her life for a while instead of telling Ash and Dawn.