Were you ever afraid of things lurking in the dark? Scared of monsters under the bed or the demons lurking beind the window? Good. Sometimes you need to be.
Cas has lived almost entire life on the road, moving from hotel to hotel for almost his whole life. He lived with a father dedicated to his work and a sister dying to follow in his footsteps. Instead of maybe working in offices everywhere for a demanding boss, Cas's father hunted down the creatures that most people chose not to believe in. Cas had counted on leaving this life one day and living the life of a normal person.
He didn't count on his sister becoming the enemy or his father going AWOL.
Now it's up to Cas to fight the good fight alone.