"21st Century Boys" is a manga series that serves as a sequel to "20th Century Boys," continuing the intricate and suspenseful narrative of its predecessor. The story picks up after the events of the original series, where Kenji Endo and his childhood friends thwarted a global catastrophe orchestrated by a mysterious cult leader known as "Friend." However, the resolution of their victory is short-lived, as new mysteries and threats emerge, challenging the characters once again.

The narrative alternates between the past and the present, unraveling the complex history of the Friend cult and its influence on the world. In the present timeline, set in the early 21st century, society has been reshaped by the cult's ideology, and a new generation of characters, including Kenji's niece Kanna, must confront the lingering shadows of the past. Kanna, along with other descendants of the original group, becomes central to uncovering the truth behind Friend's identity and the cult's ultimate goals.

Key characters from the original series, such as Otcho, Yoshitsune, and Maruo, return to play pivotal roles in the ongoing struggle against the cult. Their personal histories and relationships are further explored, adding depth to their motivations and actions. The story delves into themes of memory, identity, and the cyclical nature of history, as the characters grapple with the consequences of their past decisions and the enduring impact of the Friend cult.

The plot is marked by its intricate twists and turns, with revelations about Friend's true identity and the cult's machinations driving the narrative forward. The series maintains a tense and suspenseful atmosphere, blending elements of mystery, science fiction, and psychological drama. As the characters uncover hidden truths and face new challenges, the story builds towards a climactic confrontation that seeks to bring closure to the long-standing conflict between Kenji's group and the Friend cult.

"21st Century Boys" expands on the rich world-building and character development established in "20th Century Boys," offering a compelling continuation of the saga. The series remains grounded in its exploration of human resilience and the enduring fight against oppressive forces, making it a notable entry in the genre of speculative fiction.