Despite gestures toward defining Staten Island or illuminating the fact-fiction membrane, the latest Cinema Purgatorio release is really a scary story told in the dark, in the get-this whisper of documentary. Filmmakers Barbara Brancaccio and Joshua Zeman explore a spate of 1980s kidnappings with the inquisitive-but-we’re-good-people stance of two natives trying to get to the bottom of things. The abandoned grounds of a children’s mental hospital fuel local tale-telling centered around “Cropsey,” a generic Northeast bogeyman, until actual disappearances focus anxieties on Manson-eyed former attendant André Rand. Brancaccio and Zeman roll out citizen search groups, track court-case progress, interview twisted police detectives, dig up vintage Geraldo Rivera and Ernie Anastos clips, and spotlight Rand’s rants. By halfway through, they’ve succeeded in hedging enough that every narrator seems equally unreliable and plausible, and even the filmmakers’ true intentions remain in doubt as they play up Rand’s perpetual post-electroshock affect. Embracing what’s really standard tabloid fodder of the decade with earnest engagement and doled-out suspense, Cropsey is one step from macabre comedy. Ultimately, we’re not convinced that truth is a chimera, but just that these underdog storytellers have demonstrated an ability to tell an indeterminate yarn.