The creation of a horrible plan. People who plot will always refuse to tell any of their plans to their friends. People who plot will often come in pairs, wanting to plot together against their friends and/or partners.
“God damn it, Julia and Violet are plotting again.”
“They scare me. I think we should be concerned.”
This is a response when someone asks what you're doing and you don't want to tell, so you pretty much say you're sitting there pondering how to destroy everyone.
Rachel: Hey, what are you doing?
Tina: Just sitting here plotting against the black and white.