"Angel Beats!: Heaven's Door" is a prequel manga to the original "Angel Beats!" anime, expanding on the backstory of the Afterlife Battlefront (SSS) and its members. The narrative is set in a mysterious afterlife school where deceased individuals who experienced unfulfilled lives are sent. These individuals, known as "students," are given a second chance to resolve their lingering regrets before moving on to the next stage of existence. However, some students, led by Yuri Nakamura, form the SSS to rebel against the enigmatic "Angel," a seemingly emotionless girl named Kanade Tachibana, who enforces the school's rules.

The story delves into the formation of the SSS and its early struggles against Angel. Yuri, driven by a desire to resist what she perceives as an unjust system, recruits other students who share her dissatisfaction with their fates. Among the key members are Hinata Hideki, a former baseball player; Otonashi, a boy with no memory of his past; and Takamatsu, a self-proclaimed ninja. Each character carries unique regrets and motivations, which are gradually revealed as the story progresses. The SSS engages in various guerrilla tactics and pranks to disrupt the school's order, believing that Angel represents a higher power controlling their afterlife.

Kanade, initially portrayed as an antagonist, is revealed to be a complex character with her own mysterious agenda. Her interactions with the SSS members, particularly Yuri and Otonashi, hint at deeper connections and unresolved emotions. The manga explores themes of rebellion, redemption, and the search for meaning in an afterlife where the rules are unclear. As the SSS grows in numbers and strength, tensions rise, and the line between friend and foe becomes increasingly blurred.

The narrative also introduces additional characters, such as Naoi Ayato, a manipulative student council president, and Yui, a cheerful but troubled girl with a tragic past. These characters add layers to the story, highlighting the diverse experiences and struggles of the students in the afterlife. The manga balances action, humor, and emotional depth, providing a richer understanding of the world and its inhabitants.

"Angel Beats!: Heaven's Door" serves as a foundation for the events of the anime, offering insights into the origins of the SSS and the dynamics between its members. It expands on the lore of the afterlife school, exploring the motivations and backstories of its central characters while maintaining the core themes of the original series. The manga provides a comprehensive look at the early stages of the SSS's rebellion, setting the stage for the conflicts and revelations that unfold in the anime.