“I don’t know.” These are the final words of Sound of My Voice and might be taken to characterize the film as a whole. Peter Aiken (Christopher Denham) and Lorna Michaelson (Nicole Vicius) are lovers who join a cult in order to secretly make a documentary film about it. For Aiken, this is a kind of mission since his late mother joined a different cult with disastrous consequences. The leader of the cult, Maggie (Brit Marling, who also co-wrote the screenplay with director Zal Batmanglij), claims to have been born in 2030 and to have returned to our present from 2054 in order to save a chosen few from a dire future. Formally, this is a pretty conventional film and at first it seems to be what was called in the 1950s a problem film one dealing with a social problem. But while the film quite successfully conveys the creepiness and psychobabble of the cult, it is not ultimately concerned with examining the cult phenomenon. As the movie progresses, it seems to focus more on the character of Aiken but even this is not the ultimate payoff. This is a generally understated thriller and the key to the film’s success as engaging entertainment is its skillful use of uncertainty. Who is Maggie and what is she up to is the primary, but not the only, question posed. Uncertainty in the hands of the master filmmaker Robert Bresson is a way of exploring the human condition. Sound of My Voice does not aim so high. In it, uncertainty is merely a plot device, and an effective one. It is the primary element that makes it above average, sophisticated entertainment.