Based on The Ruins by Scott Smith, who also wrote the screenplay and is not related to the director, Carter Smith.
Jeff (Jonathan Tucker), Amy (Jena Malone) Eric (Shawn Ashmore) and Stacy (Laura Ramsey) are on vacation in Mexico when they meet Mathias (Joe Anderson) who last saw his brither Heinrich at a Mayan temple dig. As they follow him and his friend Dmitri, they end up accidentally stepping on some vines that cause the locals to grow insane at come at them with knives and guns, killing Dmitri.
It turns out that those vines, when touched, cause the natives to leave them alone. Those same vines also start to grow within their bodies, taking them over and consuming them. The vine FX are great, the scene where Stacy tries to slice herself apart to free her body of them is good but as for the rest of the movie, it’s just fine. They shot a ton of endings to this and seemingly picked the safest one. Oh well.