From the Society

Each player first notices a few strange things that occur around their homes, offices, or in their lives. Mostly innocuous, these small things are classic fairy hijinks, or bad luck, or what-have-you.

Shortly thereafter they receive large, square envelopes of creamy paper sealed with wax and an unusual seal like a knotted rope. Inside is a letter written in dark blue-black ink that explains their request has been approved and, with a probationary period beginning now and lasting not longer than until the next harvest moon, they are allowed entrée into the world of the fey.

From then on they can see the small things that act about them - fairies, ghosts, nature spirits, all that manner of things. Doors that are hidden to everyone except those with the sight, that manner of thing.

Their access to the fairy world was applied for by some figure that thinks they're the right folks to solve a particular problem.

Things get darker as they go. Starts out whimsical, then there's demons and shit. And those bad fairies.

Sources: Pact (web serial), Hellboy, Hilda, Gravity Falls, Spiderwick Chronicles, Don't Rest Your Head, Changeling: the Lost