As a child Brian was extremely sheltered by his mother to the point of it being unhealthy. So it's no surprise that as an adult Brain has isolated himself from the world - he is full of anxiety and avoids people as much as he can. He's constantly putting himself down and finding reasons why no one could ever like him.
His world is turned upside down when he finds seven-year-old Abigail hiding in the mechanical room at the school he works at as a night custodian. He notices scars on Abigail and knows she's been through more than she should at her age. Brian wants to help but doesn't feel as though he should because he doesn't want to ruin her life. But after that day Abigail becomes attached to Brian and views him as someone she can trust and eventually she helps break down the walls he has built and in turn he has helped Abigail with her abandonment issues.
I've struggled with social anxiety most of my life. It was weird reading Brian's thoughts and feelings because they're so much like my own. A very well-written page-turner.