A darkly comic thriller, Alexandre Singh’s “The Appointment” is a tale of doubling and mistaken identity that embraces the fantastical and supernatural qualities of Gothic literature, from E. T. A. Hoffmann to Roald Dahl. The protagonist is Henry Salt, an enfant terrible of letters who we meet as he wakes from a nightmare and discovers a confounding entry in his diary: “12 o’clock at the restaurant La Folie.” But who is Henry meeting? And why doesn’t he remember making this appointment? When no one appears at the scheduled time, Henry becomes obsessed with trying to uncover this person’s identity. Charging through a series of dreamlike encounters, he discovers that the truth is more disturbing than he could have imagined.