Parents Returned A Son Stolen, story of the Umi no Yari

The invasion goes south, Orochimaru summons not the first two Hokage, but Minato and Kushina. As the two parents decimate the forces of Konoha they come across the one who held the keys to their locked away freedom. They struggle to find out what has happened, who is this warrior that was their little boy? They struggle to find out who, what, when, a why they have lost their son