Nothing about this story is supernatural and it really could happen to you because it happened to them.
James Hoyt (Scott Speedman) and Kristen McKay (Liv Tyler) went to the Hoyt summerhouse after a wedding. It was supposed to be a special night but things had not gone as planned. Exhausted and drunk, the two wanted nothing more than for the night to end. Then a strange girl knocked at the door looking for someone. From that moment on, Kristen's and James' lives are in danger as they are threatened first subtly then with increasing intensity.
One of the most terrifying elements of this story is the randomness of the violence. There is nothing personal about the attack; the couple is just in the wrong place at the wrong time.
Writer/director Bryan Bertino's method of telling the story is very effective and affecting. Once introduced, the Strangers (Gemma Ward, Kip Weeks, Laura Margolis) are always lurking but not acting so the audience never knows if and when something is going to happen. There are also several long takes as opposed to a lot of short cuts. The masks add to the overall effect of fear because you cannot see the intruders' facial expressions or emotions nor are their identities ever revealed. Furthermore, the time establishing and connecting with Speedman's and Tyler's characters is well spent as you feel for them and with them in their helpless situation.
At some point, the constant appearing and disappearing of the Strangers becomes annoying but it is shortly after that that everything comes to a head. The ending, however, feels abrupt and draws attention to the shortness of the total time. On the other hand, it punctuates the senselessness of the entire incident.
Bernito creates a successful throwback to horror films of the 1970s, resulting in a movie that stands apart from other recent horror attempts.
And in case you are wondering, it would appear the story was more likely inspired by the Manson murders in 1969, the Keddie murders in 1981, and a childhood experience of Bertino, than a single more recent case as the prologue suggests.