GTO: 14 Days in Shonan

This is a spin-off story of the manga GTO (Great Teacher Onizuka) that takes place directly after the Teshigawara arc. After Eikichi Onizuka was shot by former teacher Teshigawara, he was sent to the hospital. Shortly afterwards, he escaped and spent 14 days that no one knew about. Strangely enough, when he returned, a wanted poster appeared with a man that resembled Onizuka. What exactly did he do during these 14 days? (Source: MU)