Kimi to Boku no Immoral Repeat

What if I had accepted her confession back then...? Could I have saved her?"

Hiroki Majima, a plain and pathetic middle-aged man, is overwhelmed with regret after learning of the death of his first love, Ryouko Tokiwa. Lost in his remorse, he suddenly finds himself transported back in time to 1997, at precisely the moment when a young Ryouko is confessing her feelings to him.

Without hesitation, Hiroki accepts her confession and vows to make the most of this second chance at youth. But what began as a dream come true quickly turns into a nightmare when a tragic event drags him into a dark, controversial, and utterly immoral mission.

(Source: Kadokawa, translated)