[BladeStone]

Summaries of books II, III, and IV (translated from Japan's Wikipedia):

Spoiler for Book I (for perspective):

Barusa, a female bodyguard, is accidentally entrusted with the task of protecting Chagumu, the second prince of the New Yogo Kingdom. Lodged in his body is the egg of the water fay Nyunga Ro Imu (the guardian of the water) from in the other world (Nayugu)*. That other world overlaps with this world (Sagu). Chaguma is pursued by his father the King, who tries to kill his son in secret in order to save his prestige, and by Rarunga, a monster from Nayugu** which eats the egg of Nyunga Ro Imu. Barusa, who escapes with Chagumu from the palace, begins to live with Chagumu along with her childhood friend Tanda, a magician, and Torogai, Tanda's master, until the egg leaves the body of Chagumu at Summer Solstice. Barusa has also had the experience of being pursued by killers and leaving her homeland when she was young, and she was saved by a close friend of her father, Jiguro, a master of the halbert. In the meantime, the astrological scholar Shuga comes to know that the fay's eye lodged in Chagumu is a monster that is said to have been defeated by the founding King of the New Yogo Kingdom in the past, so he begins to search past records. And Shuga learns that the legend of Torugaru has been falsified, and that Nyunga Ro Imu is a being that allows the sky to rain and aids the growth of crops.

Spoiler for Book II:

Barusa, having discharged her task of proteching Chagumu successfully, returns to her homeland, the Kanbaru Kingdom, in order to pay her respects to the grave of Jiguro. When Barusa was young, her father who was the principal medical doctor of the King was forced to poison the King by the threats of the King's younger brother Rogusamu. Afraid that he and Barusa would be silenced by murder, he asked his close friend Jiguro, who was said to be the best halbert master in 100 years, to escape with Barusa. From then on, Barusa led a fugitive life ith Jiguro until the death of Rogusamu. She learned to use the halbert and survived. After Jiguro died, Barusa became a bodyguard. She decided that her debt [to the eight people who died protecting her] would be settled with her protection of Chagumu. However, upon her return to Kanbaru, her learns that Jiguro was charged with the infamous crime of stealing the golden wheel, a national treasure which is used at the coronation ceremony. Barusa saves Kanbaru which is surrounded by darkness.

Spoiler for Book III:

Torogai is the master of Tanda, Barusa's childhood friend. Torogai used to be disappointed with her predictable and depressing life as a daughter of peasants. She fell in love with a man in a palace by a beautiful lakeside in her dream and gave birth to a child. Barusa saves Yuguno, a singer who is pursued by abductors. Yuguno is loved by Lii ('the wood spirit'), and is Li Tou Ruen ('the lover of the wood spirit'). Chagumu is depressed at becoming the crown prince due to the death of the first prince. There is no reason why Chagumu who has met Tanda and Barusa and come to know the warmth of ordinary people would like living in the palace, still less reason for him to respect the King who has tried to murder him. Hearing Chagumu's complaints everyday, Shuga tells Chagumu that he has been studying spells outside from Torogai in secret. Chagumu, who misses [Barusa & co.] a lot, is attracted by a voice in his dream and does not wake from his sleep. In the meantime, there are others who do not wake from their sleep as well, such as the First Royal Concubine who has just lost her son the first prince. Tanda's elder brother's daughter Kaya also does not wake from her sleep. Tanda tries to help Kaya with his spell but falls into a trap of the enemy. Although Tanda's body has been taken over by 'the Flower', he has somehow managed to retain control of his mind and finds Chagumu in his dream.

Spoiler for Book IV:

Barusa accompanies Tanda to Yogo's yearly herb market in autumn. They visit an inn town near the border of the Rota Kingdom. There she meets an elder brother and younger sister who have been brought there by slave dealers. They are the children of the Taru people who are abhorred in Rota. The slave dealers who are staying at the same inn as Barusa and Tanda by chance are torn into pieces by some unseen being. The elder brother Chikisa and Barusa are injured, while the younger sister Asura has fainted away but is unharmed. The inn is consumed in fire. Barusa saves Asura who has been captured, and Tanda and Chikisa are caught. It turns out that the one who is pursuing the siblings are Tanda's acquaintances, Sufaru*, a wizard from Rota, and his daughter Shihana. A long time ago, a daugher of the people of Taru becomes possessed by the blood-thirsty demon Taruhamaya, and controlled the entire Rota under her tyranny. Since then several hundred years have passed, the terrible demon has reappeared briefly through the young girl Asura, so Sufaru tells Tanda. Therefore, Asura must be disposed of. However, those who long for the second advent of Saada Taruhamaya are enclosing the Rota Kingdom in a web.