Soma is a forbidden technique used to change a dead body into a zombie who follows their master's orders.

All of her life, Rinka was imprisoned by her father in her room and only accompanied by her father's soma, Yan-yan, as her caretaker. She always wanted to go outside to search for Hourai, a place where fish swim in the sky, birds fly in the water and waterfalls flow up to higher ground.

One day, she couldn't wait anymore and with Yan-yan's help she escaped from the room. They both went outside. And then, their journey to find Hourai began.

(Source: MyAnimeList.)

(For more information in Japanese, see the official page of the first book at Kodansha.)