Junta has a problem: he's deathly allergic to girls! All this changes with the arrival of Karin,​ a gorgeous time-traveler who transforms him into the Mega-Playboy,​ the sauciest girl-magnet around.​ Suddenly,​ Junta's wildest dreams have come true,​ and all the prettiest girls in school have fallen in love with him.​ With so many adoring fans,​ mayhem is sure to follow,​ and Junta finds himself in the center of one hilarious romantic mishap after another.​ 
 
(Source: Discotek)

Japan in the future has a massive over-population problem. Scientists have traced the root of this problem to one particular individual, Junta Monamari. According to their records, Junta was a "Mega-Playboy" who sired children with over a hundred women. Each one of his sons inherited the Mega-Playboy gene too, so the problem grew exponentially from thereon.
 
Meet Karin Aoi, a "DNA Operator." Karin is sent back to the past with a mission: to alter Junta's DNA and prevent him from ever becoming the Mega-Playboy, thereby eliminating the world's overpopulation problem. Junta's DNA can only be altered by shooting him with a special bullet called DCM which contains the chemical for suppressing the Mega-Playboy DNA. In return, Karin gets a vast sum of money... enough to make all her dreams come true. Will Junta become the playboy he is destined to be, or will his better self prevail? But Karin is falling for the real Junta too... and what of the sweet and kindly Ami who has secretly loved Junta all her life? What will happen to future Japan?
 
(Source: Manganews)

Junta's great granddaughter goes back in time in order to make Junta stay as the Mega-Playboy so she can continue to exist. Along with her comes an old man with the intention of making Junta stay as the Mega-Playboy forever for his own purposes.When Junta starts getting attacked, he has no choice but to return to being the Mega-Playboy in order to protect his loved ones.

Chiyo Momonari