越战英雄军官金镇平（宋承宪 饰）新接收了一个部下荆宇真大卫（温朱万 饰），荆大卫的妻子钟佳欣（林智妍 饰）也随丈夫一同住进了军队官邸。镇平为佳欣的魅力所吸引无法自拔，以致发展出一段军中不伦之恋。该影片由执导过《方子传》、《淫乱书生》等影片的导演金大宇导演执导。一直以偶像形象示人的宋承宪首度挑战大尺度裸露戏份。
故事设定在1986年，即《招魂3》的五年后。沃伦夫妇因艾德在第三部中经历恶灵事件而突发心脏病，已正式退休，不再从事驱魔工作。他们仍会到大学巡回演讲，但连这类机会也在逐渐减少。一些事件最终还是让他们重返战场。这次的核心案件，是沃伦夫妇生涯中最著名的一起事件之一：斯莫尔家族闹鬼案。据“新英格兰通灵研究协会”（由艾德与洛琳的现实中的女儿朱迪和女婿托尼·斯佩拉负责运营）称，1970年代，珍妮特与杰克·斯莫尔搬入宾夕法尼亚州西皮特斯顿查斯街上的一套复式住宅。随后数年，包括他们的女儿与杰克的父母在内的家人纷纷声称经历了超自然现象，从怪味、怪声到鬼魂骚扰等。
This long-lost masterpiece of hardcore gay erotic cinema centers on a handsome young stud who rides his motorcycle through myriad sexual encounters, from a soccer game’s locker room to a phantasmagoric and deeply melancholy orgy. A labyrinth of all-male fantasies whose outstanding beauties always end up fading away while leaving their sensual protagonist dealing with his own loneliness, EQUATION was directed with supreme grace by the mysterious Dietrich de Velsa (aka Francis Savel/Frantz Salieri). A former painter, de Velsa was also the owner and artistic director of La Grande Eugène, one of the first drag cabarets in Paris. Years later, he collaborated with Joseph Losey on MR. KLEIN (1976) and DON GIOVANNI (1979). His one and only film, EQUATION TO AN UNKNOWN stands as one of the most extraordinary erotic films of all time. “The most melancholic porn film I’ve ever seen. It’s very sad, and aesthetically beautiful. Just a young man wandering through several depictions of love and choreographies of love-making; it’s like a ceremony, a ceremony of fantasies, especially the last sequence where the main character recalls all his previous fantasies, which come one by one around his bed and make love to him. I think it’s one of the most beautiful film sequences of French cinema.” –Yann Gonzalez
