...a pencil and paper.
When I have a plot point that I'm having trouble with, there's nothing worse for me to do than sit in front of the keyboard willing an answer to appear.
In such circumstances, I can do nothing better than go to my local coffee shop (Costa, in this case) and sit with a pencil and paper to work it out.
There's a sense of mental freedom that I get with paper that just isn't there when I'm in front of the laptop.
So, as you can guess, I'm in one of those states right now. I've got a tricky plot point in my current short story.
Excuse me while I sharpen my pencil.