Before I Knew You tells the unsettling story of what happens when two very different families swap houses across the Atlantic one August. William and Beth, newly-weds from Connecticut, need to visit London so that William can spend time with his three sons from his first marriage, while Sophie and Andrew, who have two teenage daughters, are looking for a holiday that might rescue them both from the daily grind and a gradual disintegration of their relationship. Neither family finds what they expect. Strangers when they borrow each other's respective homes, their lives then start to intertwine in unexpected ways, throwing them at the mercy of their pasts and each other.