On a warm summer day, six-year-old Danny Mercer races into the street to retrieve a soccer ball when the unthinkable happens. With the screech of tires and a sickening thud, his world and that of his family shifts on its axis. Doctors deliver a shattering diagnosis in the form of an acronym: TBI. Traumatic Brain Injury. As months pass, with Danny in a prolonged coma and hope for recovery fading, the emotional trauma to his family escalates. Desperate to heal their son, parents Colt and Rachel Mercer turn to doctors Jon David and Sara Luke, a husband-and-wife medical team, for help. Their audacious plan for his healing will test the limits of the Mercer's faith and belief in God. Knock on the Sky is an intimate, soul-searching tale highlighting the surprising arcs our lives may take to discover Truth and spiritual meaning.