When you're touting "shot on an iPhone", better make sure it's a surprise, not an explanation. Sadly, Threshold is the latter. A film that positively looks like it was shot on a phone. It's also structured like one, with a lot of dialogue and drama acting as filler for the minor mystery and horror elements that hope to draw in the crowds.
Thinking his sister Virginia is back to doing drugs, Leo sets out to find her. When the two are finally reunited, Virginia tells her brother she's been clean for 8 months straight, but a weird cult hexed here. Though Leo doesn't believe a word she says, he wants to support his sister, and they drive across the country to try and find the cult that tool a hold of Virginia.
There are three noteworthy horror/thriller moments, the rest is just two siblings reconnecting after not having seen each other for a long time. The performances are rather weak, the cinematography is shabby and even though the film is short, the middle part really drags. There are glimpses of quality if you look long and hard, but it's not enough to keep this film afloat.