Parasyte -the maxim-

Shinichi's life returns to "normal" after A's killing spree, but Shinichi remains in low spirits. His mother, Nobuko, becomes worried, but Shinichi just bluffs when she asks him what's wrong. His father, Kazuyuki, suggests to Nobuko that a past incident may be involved. Nobuko looks down at the old scar on her hand and thinks back to what happened when Shinichi was a little boy...