Tomoe Saki is a weak and cowardly girl who only learns karate. But one night, while going back home from school, she rescues a mysterious sunglasses person from a group of thugs. The mysterious person gives Tomoe a yo-yo and chooses her as the new "Sukeban Deka". As Tomoe feels complicated by her new position, the yo-yo teleports her back to the Showa era in the '70s, where she encounters the original Sukeban Deka, Saki Asamiya, and helps her take down some goons. Now Tomoe must go back and forth from the past to present and help Asamiya solve various cases that reflect on the present, while trying to decide if she is worthy to take on the mantle of becoming the new Sukeban Deka.