"GS Wonderland" is about the 1960's Group Sounds bands - A Japanese pop movement inspired by the Beatles and other mid-1960's Brit Pop. Three young men set out to start their own band "The Diamonds" when they are signed to a recording contract during their very first rehearsal. Things turn for the unexpected when their agent brings Mick (Chiaki Kuriyama) into the group as their keyboardist.
A girl (Anna Yamada) is just 14-years-old, but she endures a difficult life. Her parents abuse her at home and she is bullied by students at her school. She doesn't have a place where she can feel comfortable. One day, the girl is kidnapped by a man (Shuhei Uesugi) wearing a mask. The kidnapper names the girl Sachi, while wishing her to be happy. For the first time, Sachi can feel happiness.
A chance meeting between university student Isumi and police officer Amasawa leads to late-night meals and unexpected feelings, challenging their perceptions of love.
