Jimi na Ojisan, Jitsu wa Eiyuu datta. ~Jikaku ga Nai Mama Musou Shitetara, Mei no Dungeon Haishin de Sarasareteta you desu~

Keita Sato (41, single) is overworked at his corporate job, dealing with tedious and stressful tasks day in and out. How does he relieve his stress? By sneaking into modern-day "dungeons," using them as a batting cage, and blasting away ferocious monsters! One day, his live-in niece Hikari secretly follows him and livestreams his adventures, only for his unbelievable strength to go crazy viral!! Keita unknowingly captures the attention of not only viewers, but other big-name streamers.