After the appearance of a mysterious Tower, the world slowly returned to normal. However, every year, five people are forcibly chosen to enter the Tower, which has a thousand floors. Not a single person has ever managed to return. Reo, a convenience store cashier who often watches livestreams of Tower explorers, never imagined that he would become one of the people involved in its mystery. After a tragic incident, Reo opens his eyes and finds himself inside a cave. His body has transformed into that of a young girl named Rosel. Now, she is on the first floor of the Tower. Inside the Tower, every floor has its own kingdom, races, monsters, nobles, rulers, and bosses. Rosel only wants to understand what happened to her and find a way out. But there is one great secret she does not know. Rosel is actually the Floor 1000 Boss. The mysterious figure who has never been seen by anyone. The existence feared even by the other floor bosses. Strangely, Rosel herself has no idea about