Imprison Fate arc
Mahora Martial Arts Tournament Arc
Investigation arc is the fourth story arc of the UQ Holder! series.
Touta's desire to train is halted by Yukihime as she gives him a job to investigate a dangerous immortal. Together with Kuroumaru, Karin and Ikkuu, they are to infiltrate a private boarding school in order to recruit him.
Training in the morning, Touta recalls the recent events. Intending to get stronger, Touta reaches Yukihime with a request to be his coach. After talking about Fate, as well as UQ Holder, Touta receives a job to recruit an immortal and if that were to fail, to dispose of him. He is joined by Kuroumaru and Karin, putting on their uniforms in order to infiltrate a private boarding school as students. Much to Kuroumaru's embarrassement, he is given a girl uniform by Karin. In the middle of their banter, Ikkuu joins them and soon, the group arrives at Amano-Mihashira Academy City.