Todd (Tyler Labine) is a bit of a pushover who is planning on proposing to his strong-willed girlfriend Cammie (Malin Akerman) at his parent's cabin. His brother Sal (Dan Petronijevic) and Sal's girlfriend Masha (Lucy Punch) show up unexpectedly. They are loud and rude and are ruining Todd and Cammie's weekend.
Cammie and Masha go to pick mushrooms so Todd can get Sal to leave after Sal mentions a party he wants to throw. The latter two get into a fist fight after Todd throws Sal's headphones into the lake which results in Todd killing Sal with an ax to the neck.
When the girls get back, Todd tells Cammie what he did and she coerces him into killing Masha as well (Cammie hits her with a frying pan and then strangles her). They chop up their bodies and dispose of them in the lake, after which Cammie accepts Todd's proposal.
When they return to the cabin, there is a party going on. Sal's friend Dov seems really suspicious of Todd and Cammie and ultimately, after he finds the headphones, is killed by Todd and Cammie with an arrow to his eye and heart.
A policewoman arrives, having received a message from Dov the evening before. Todd's parents arrive, with Todd's mother being a pushover and Todd's father being aggressive. They find blood on the property and a sleepwalking incident from Cammie makes Todd's mother think they killed Sal.
Todd and Cammie blame a drifter they saw earlier who gets arrested. After Cammie insists their wedding be held in the same cottage all the murders have happened in and she makes him yell at his mother, Todd realizes he doesn't love her and needs to turn himself into the police. He takes off in his car with Cammie (who agrees she doesn't love him) in hot pursuit.
As he makes it to the police station, he gets out of the car to approach the policewoman from earlier. Cammie speeds towards him and the policewoman takes her gun out and shoots Cammie in the chest. Cammie swerves and crushes the officer. She then takes the dropped gun and shoots Todd in the head, takes off her engagement ring and dies.
Epilogue shows Todd alive, in a wheelchair with his father (who reveals his wife has left him). Todd says the lake is pretty, then asks about his mother and brother. He repeats the same questions a few times, revealing that even though he is alive, the bullet has caused irreparable damage.