Most of the story takes place inside the mansion where Finn encounters a lot of ghosts and he has to steal the pocket watch from Sir Clockwood. This mansion is one that Finn had invaded, home of lots of the ghosts and spirits.
Finn wakes up on the porch of the mansion where he has no memory of anything of his past besides his name.
Finn is forced to enter the mansion because he refuses to stay outside in the rain and get cold. He enters the mansion and he sees a reflection on the puddle in the group and quickly ducks seeing and knowing the sound of something flying, a knife to his head.
Unfortunately, Finn is forced to sleep under the staircase for the sake of safety. He gets the opportunity to talk to one of the ghosts about the history of the mansion and what they've all done. During this time Finn observes a key pattern.
I need to get that pocket watch...
We've all been here for such a long time and Clockwood doesn't let us leave... that stupid pocket watch of his...
Finn had previously realized he has to get his pocket watch in order to be freed of the mansion and to help get his memories back so when he sees Sir Clockwood, he has to trick him into getting the pocket watch.
Finn had the pocket watch and all ghosts and spirits were freed allowing them to Rest In Peace, and Finn leaves in search of finding out how he really got there with little of his memories back.