Ever since she was a child, Kei Yonagi has been attracted to the idea of acting, as she has always attentively watched how actions and emotions differ depending on the type of character portrayed. Now a teenager, she is broke, and is the only one able to financially support her young siblings. She starts auditioning for any role possible, without much success.

Her luck begins to change when Sumiji Kuroyama, a renowned director, sees Yonagi's performance and is amazed by her ability to immerse herself in the role—the so-called method acting. Although she is a diamond in the rough, if she keeps using the same technique over and over, it could lead to her being severely damaged. So, he makes the decision to help polish her, giving Yonagi the opportunity she needs. However, the road ahead of her is far from easy, and she will need to adapt to overcome the various challenges that come her way.

As a part of the JUMP START initiative, the first three chapters of Act-age were published in English in VIZ Media's digital Weekly Shonen Jump on the same day as the Japanese release. The series was discontinued after the arrest of storywriter Tatsuya Matsuki on August 8, 2020.