The plot will always be agent S's plot indefinitely because if you invited her into town, it's a set move-in so she'll move in no matter what. For plot resetting, because agent S is moving in tomorrow, you want to make sure the game is set to tomorrow.(only her plot will show up, not her actual house until the next day) You can do this by changing the 3DS clock in settings. Don't change the clock via the in-game clock because this will load up the mayor. Here's why:
You
don't want to load up the mayor because once you load up the mayor, the position of her house will be saved and
it is pernament. So what you do want to do is "create a new save file" which can be found within the list of options in the start up screen. If you create a new save file, this means you can use it to your advantage. Because they aren't already a permanent resident in town this means anything they do won't be saved, because you won't be able to load them into the game you have to create them first. Unless you go to Isabelle and put their house down and save it and so on.
Once you've got your new character, just look around town for Agent S's plot. Once you find her plot, if it's not somewhere you'd like it to be exit the game using the home button and repeat the process again until you find a plot you do like. When you find a plot that you like, go to Isabelle and she'll tell you that you need a house in town before she can register you as a resident. After you head to Nook's homes and plot down a house, then head back to Isabelle and she'll register you as an official resident.
After that the game will save, therefore Agent s's new plot will be permanently placed where you want it to be. After this, you can delete the character as you won't be needing them any more, you can do this by clicking the characters name on the load up screen and selecting the "I need some help first" option from Isabelle and then say "demolish my home" or something like that and it'll delete the character.
Just repeat this process for all plot resets that you do. Good luck!
