Lindsay was always afraid of Monsters; Monsters under her bed, monsters in the closet. Usually when you 'grow up' you loose those fears, realising they are completely irrational, but at the age of 15 it became worse than ever.
Lindsay, along with the rest of the small town she lived in realise that there was a whole other kind of monster. One that was out there, and always will be out there, no matter what anyone does.
This kind of monster doesn't pull you under your bed and eat you, but it does something horribly worse. It destroys people, families, thousands of people's lives for no reason at all.
This monster isn't green. It doesn't have thousands of eyes, or sharp pointy teeth, but instead looks just like us, which is one of the scariest things about it. You don't know who the monster is. It could be a co-worker, a friend, and relative or even just a stranger walking down the street. The only time you realise is when it's too late.
Lindsay had experienced this monster for the first time when she was fifteen. That monster had come and taken her best friends from her. And what was worse than the immense sadness and grief that was left because of the monster was the guilt. The "Why wasn't it me, why was I the only survivor?"
That's why Lindsay does what she does. She was never able to save her friends, or catch the monster that destroyed them, but maybe, just maybe she could help catch someone else's monster.