Naruto: Itachi Uchiha

As Itachi Uchiha witnessed the death of his closest friend, Shisui, the overwhelming grief awakened his Mangekyō Sharingan.  But that was not the end. Unable to bear the crushing pain, his mind was trapped within a genjutsu by Shisui's Mangekyō. At that very moment, a genius from Earth met his own death and found himself reincarnated inside Itachi's body.  After breaking the genjutsu imprisoning Itachi's consciousness, the two shared each other's memories, gaining complete understanding of one another's lives.  Realizing the newcomer possessed the resolve and intellect to change fate, Itachi willingly entrusted him with his body, choosing to remain as a silent observer within his own mind. Thus, a new Itachi Uchiha was born: one who retained Itachi's body, talent, skills and possessed a mind even more brilliant and far more dangerous than the original.  One who would not be held back by a normal Mangkyou Sharingan, average chakra reserves and being played by others like Danzo. - MC has only watched Naruto, not Shippuden - Original story, not a translation - From Strong to Overpowered