About twin sisters married and they and their husbands go on a weekend getaway with a 5th wheel (male friend). The 5th wheel had been having an affair with one of the sisters while her husband was deployed for the military.  The 5th wheel keeps a box with photos and has flashbacks to his childhood when his sister drowned. The military husband starts shooting people. Turns out the sisters soul of the 5th wheel went into his body.