Imagine being in a room with someone.
You absolutely know they're there, they're not invisible, but you can't see them. The best you can do is to catch small, quick glimpses from the corner of your eye that don't allow you to confirm the person's height or coloring or even if they're male or female. Every time you try to get a look at them, you're compelled to look away so that even sneaking a peek becomes a chore.
This is what happens when someone is Unnoticeable, and people who fit that description are the Wardens' secret weapons.
Unnoticeables can be sent in to do a job, and no one will remember their presence, so how better to carry out clandestine midnight maneuvers that can never be acknowledged? How better to round up touched children and their families in the middle of a bustling neighborhood with nosy neighbors than to send in someone those neighbors wouldn't bother to wave at if by some fluke they saw them on the street? How better to keep secrets from supervisors who think they're the ones wielding power?
Penn, the main character, is used to illusions, but even circus life and a knack for spotting the unseen mechanism behind a trick isn't enough to fight technology that can literally blot someone out of existence.
How can you run from someone when you don't know how many people are chasing you?