The Labyrinth is an expansive space deep underground where humans live in clusters known as colonies. Over the years, the surface has become a distant memory—even perhaps only a fantasy to those who have never experienced its wonders. Making sure humanity survives the harsh conditions of the underground, a colony's citizens can take on a variety of specialized jobs. These include Workers, who mine precious ore to fuel the colonies, and Markers, who journey into the Labyrinth's surprisingly lush environment to bring back information that eases navigation. However, humanity also faces a threat to its existence—creatures called kaijuu whose sizes range from that of a small child to an enormous building, and are hostile to any human they see. Moreover, kaijuu that are large enough can force their way into the colonies, further increasing their threat level. Memenpu is a nine-year-old college graduate whose inventions have greatly benefitted the Workers in her local colony. Recently, however, she has been dreaming of a place with a neverending ceiling not bound by bedrock. These aspirations fuel her desire to become a Marker and explore the Labyrinth's vast unknown in search of such a fantastical place. Despite her father Gagumber's vehement disagreement, a certain incident with the kaijuu jumpstarts a dangerous yet exciting adventure that will surely alter humanity's course forever.

The Achiga Girls' Academy in Nara once defeated regional mahjong powerhouse Bansei High School. It advanced into the national team semifinals but lost to the eventual champion, and the mahjong club was later disbanded. Six years later, elementary school student Shizuno Takakamo befriends transfer student Nodoka Haramura. The two eventually enter Achiga Girls', but Nodoka transfers out of the school in the second year. When Shizuno sees Nodoka on television the following year as the national middle school individual mahjong champion, she decides to revive Achiga's mahjong club.

Due to childhood circumstances, first-year high school student Saki Miyanaga hates anything related to mahjong. When she does play the game, she keeps a low profile to avoid standing out. One day, her friend Kyoutarou Suga drags her along to the school's mahjong club. There, she meets Nodoka Haramura, the champion of the previous year's national middle school mahjong championship, and Yuuki Kataoka, a fellow club member. As the club play a few rounds, Saki attempts to hide her talent. However, the club president, Hisa Takei, notices Saki's unusual playstyle. Realizing her potential, she puts Saki's real ability to the test—proving her to be more talented than expected. This revelation damages Nodoka's pride, and their relationship gets off to a rocky start. Saki ends up joining the club, and after clearing up the misunderstandings between her and Nodoka, friendship blooms between them. As Saki slowly warms up to mahjong once again, she and the rest of her club only have one goal—to win nationals!

The Blue World, which symbolizes reason, is under constant attack by the creatures called the Nightfly O'Note originating from the Red World, symbolizing instinct. Akira Asagi, the 17-year-old protagonist who is a researcher at Blue World, is nominated to lead the war at Ryuukyuu LAG, a proud defense facility on the islands of Ishigaki and Iriomote. She meets the sixth combat unit IS, which was formed four years after a fierce battle that annihilated the fifth unit. Akari must lead these six riders, also called the Scared Rider Xechs, to fight against the other world while at the same time deepening the relationship among them. (Source: MAL News)