Supernatural (TV series)/Recap/S07/E19 of Grave Importance

Season 7, Episode 19:

Of Grave Importance
Sam and Dean get a call from Annie Hawkins, an old hunter friend, asking for help on a case. When they arrive in town, they find out Annie has disappeared. They trace her last whereabouts to an old abandoned house that is haunted by a powerful ghost. Unknown to them, the ghost of Bobby is following them around, having tied himself to his old flask that they carry. He finds the house populated by mostly mindless ghosts and the spirit of Whitman Van Ness. Bobby discovers the ghost of Annie and the two try to solve the murders.

Tropes

 * Fate Worse Than Death: Bobby and Annie discover that, after time, ghosts develop a kind of alzheimers and eventually disintegrate into nothing. Loudly.
 * Haunted House: The Van Ness House.
 * Invisible Main Character: Bobby hasn't managed to master the tricks to being a ghost, so he has difficulty alerting the Winchesters to his presence. This is particularly frustrating to him.