Intrepid adventurer Bell Cranel has leveled up, but he can’t rest on his dungeoneering laurels just yet. The Hestia Familia still has a long way to go before it can stand toe-to-toe with the other Familias of Orario — but before Bell can set out on his next mission, reports of a brutal murder rock the adventuring community! One of Bell’s trusted allies stands accused of the horrible crime, and it’s up to Bell and his friends to clear their name and uncover a nefarious plot brewing in the dungeon’s dark depths.

A demon has emerged deep beneath Tokyo. The governor of Tokyo and his escort unit go to inspect but are trapped underground. Surrounded by darkness with nowhere to go, people lose their reason and become consumed by the demon.

In the year 2062, Earth finds itself fiercely battling Artemis—an AI which rebelled against humankind following a grand-scale workforce automation project failure years prior. In the hope of winning the war, a group of three highly skilled human warriors is assembled, known as the guardians of humanity. Empress, the last surviving guardian, wakes up alone in an empty research facility. Unable to recall anything from the past except a place called "Lighthouse No. 8," she begins to search for answers. It is not long before she discovers an impending threat from Artemis—and she is the sole person who can save the world from destruction.

In 2019, following an uprising in the USA and a subsequent global nuclear war, humanity was decimated by a catastrophe called "The Maelström." A half-century after this cataclysmic event—to preserve humankind—the World Health Organization (WHO) has progressively assimilated worldwide political power and completely redefined society: people are now considered public resources and are guaranteed a happier life. The heightened development of medicine, nanomachine technology, and augmented reality promote this greater devotion to human health. Suffocated by the excessive kindness of Japanese society, high school student Miach Mihie decides to oppose the societal values she considers dehumanizing before the health implant surveillance system WatchMe activates itself the day she turns 18. Miach plans to kill herself with two of her friends, Tuan Kirie and Cian Reikadou. However, Cian betrays the pact in disagreement with Miach's intentions, allowing Tuan to survive her suicide attempt, unlike Miach, who is declared dead. Thirteen years later, now a high-ranking inspector of Helix—an organization affiliated with WHO to monitor military conflicts—Tuan is sent back to Japan after her drug smuggling activities in the Niger warzone are discovered by senior Helix inspector Oscar Stauffenberg. On her first day back to the country she tried running away from, thousands of people simultaneously commit suicide. Tuan is now in charge of investigating this wave of mass suicide and exposing the terrorist organization behind the unprecedented attack.