One of My Sons - Anna Katharine Green

A young girl frantically summons a gentleman walking by on the street to come in and help her grandfather. Arthur Outhwaite answers her cry for help only to find himself as the last person to see her grandfather alive and left with the admonishment from the dying man to deliver a letter to someone, and to that person only. Unfortunately, he dies before he can inform Outhwaite who that particular person is. Being in a house of strangers, Outhwaite is thrust into the mystery of not only finding this unknown person, but is also faced with the mysterious circumstances under which the child's grandfather died. (Summary by Laura Victoria)