Randolph Tiefenthaler's life seems to find fulfillment with the purchase of a beautiful old apartment in Berlin. The architect and his family suspect nothing when the quirky Mr. Tiberius leaves cake at their door. But soon, the neighbor from the basement becomes unsettling. He watches Tiefenthaler's wife, writes increasingly love-struck, then defamatory letters, and even files a complaint. The marriage plunges into a crisis, and the mere existence of the neighbor poisons their daily life. The unbearable situation leads Randolph to a horrifying thought...