Dear J tells the story of an evangelist, James Jamisin (Joseph A. Halsey), who has committed himself to a mental institute. He is trying to cope with the unexpected death of his agnostic girlfriend, Paige (Maya Serhan). To sort things out, he replays in his mind a make-believe trial, presided over by a no-nonsense judge (Karen Lynn Gorney). Dr. Frolick (Allison Lane) is a rookie psychologist assigned to James' case. As she seeks to unravel clues to his mental state, she realizes his path to release lies within the still unopened letter from his deceased girlfriend. The director of the institute, Dr. Donovin (Carson Grant), begins to resent the seeming attraction between Frolick and her patient. When Frolick takes a leave of absence, James' only source of counsel lies with the institute's janitor, Staples (Patrick Mitchell). However, it is only when James is finally able to summon the courage to open his letter that he can take the key that will release him from the prison.