Newcomer Brit Marling was big news at last year's Sundance film festival with two indies, co-writing and starring in both. One was an intriguing, if somewhat anaemic sci-fi concept called Another Earth, about the discovery of a duplicate planet Earth up there in the heavens. It was a film that exasperatingly failed to deliver on its promise, featuring a somewhat self-conscious performance from Marling. More or less the same can be said of Sound of My Voice. This too has a very interesting idea to start off with. Two documentary film-makers, Lorna (Nicole Vicius) and Peter (Christopher Denham), infiltrate a cult led by Maggie (Marling), a fey blonde who claims to come from the future. She's a scam artist … or is she? Some nice twists promise us an old-fashioned, well-made thriller. Infuriatingly, however, it doesn't arrive, and a plot strand remains untied; we are left feeling this residual mood of vague ambiguity and disquiet is supposed to make up for it.