Dragon Ball Z Special 2: Zetsubou e no Hankou!! Nokosareta Chousenshi - Gohan to Trunks

Sinopsis

It has been thirteen years since the Androids began their killing rampage and Son Gohan is the only person fighting back. He takes Bulma's son Trunks as a student and even gives his own life to save Trunks's. Now Trunks must figure out a way to change this apocalyptic future. (Source: ANN)