As Johnny Dark tells it, in 1967 he approached a young playwright named Sam Shepard in a New York City drugstore. “I saw your play last night,” he told him. “I was curious what drug you wrote it on.” The two became fast friends. Then, when Shepard married Dark’s stepdaughter, they became family. Treva Wurmfeld’s documentary Shepard & Dark, which opened in theaters on September 25, focuses on the nearly 50-year friendship between the men and their decision to compile their extensive correspondence for this month’s book Two Prospectors (University of Texas Press). The story of Shepard and Dark is more than just a portrait of an artist and his confidant—it is an examination of the forces of male friendship and just how much such a relationship can withstand.