She works for Gardenia apparently, I don't know who the heck this chick is. Must have sommething to do with the League. I mean, there's a picture of Gardenia, Lucy, and Cynthia down in ther corner. And there's that other chick. Francesca? Who the heck is that? No one cares, because Gardenia, Lucy, and Cynthia are total hotties, and Francesca is a wannabe. Stupid Francesca. You'll never be able to look good in a crop top like Lucy can.
Anyway, Theresa, right. I did some research, and by that I mean I went to Bulbapedia and looked her up, and apparently she has nothing to do with anybody. Her poor old granny's Pikachu ran away and so she decided to borrow Ash's, because he's the only person in Sinnoh who has a Pikachu, because no one in Sinnoh wants a Pikachu. If some one wanted a Pikachu, they could just walk over to Route 205 and find a Pachirisu, because Pikachu is not worth it. Neither is Pachirisu for that matter, but that's beside the point.
Anyway, Ash, being Ash, lets this total stranger borrow his best animal friend for a couple days. Good idea buddy. I can see why Charizard loved you so much. So yeah, Granny Smith starts using Pikachu to singe berries off of trees, because that's eco-friendly, and then Ash and Theresa start worrying if that's a good idea, probably because that could start a forest fire. And then Granny challenges Ash to a battle. What?
And then Granny tells Ash she knew that wasn't her Pikachu all along. You suck Ash.
Then Team Rocket shows up, and that goes exactly as everyone expects, and then Sugar, Granny's Pikachu, now a Raichu somehow, shows up, and everybody's happy. Yay.