Ichigo Kurosaki vs. The Assassin is a fight which takes place on the roof of Rurichiyo Kasumiōji's home.
The strange young girl from the day before mysteriously shows up in Ichigo Kurosaki's class dressed as a student. She is surrounded by the older men from before, who are also wearing uniforms in order to blend in with the other students of Karakura High School. Introducing herself as Rurichiyo Kasumiōji, the little girl names her companions as Kenryū and Enryū. Only speaking with Ichigo, she initially wants to walk home with him and his companions, but Kenryū bribes her with the promise of sweets to abandon this.
That evening at dinner, Rurichiyo appears smitten with Ichigo, much to the bemusement of Kenryū and Enryū, whom she tells to stop worrying about her so much. At Ichigo's, Rukia Kuchiki, Ichigo, and Kon discuss the three newcomers and how everything surrounding them is a mystery. They decide to sleep on it, but later Ichigo and Rukia feel the Reiatsu of an adversary close by. Upon investigating, they, discovering Kenryū upon his roof dressed as a Shinigami, initially believe it belongs to him. Before anything can be discussed, a bright light appears as an intruder forces its way through the roof. The three of them investigate, but Kenryū, jumping out of a window, runs away with Enryū, who is carrying Rurichiyo on his back.
Trying to evade Ichigo and Rukia by heading to the roof and towards Rurichiyo, the intruder is pursued by both of them. As Rukia surmises tthe others are clearly his target, Ichigo, agreeing, swings his Zanpakutō towards the intruder, who blocks with his own weapon, as Ichigo demands to know what he wants with the young girl. The mirror on the intruder's weapon uses light to blind and stun Ichigo so he can defeat him. As Ichigo recovers just in time, with his sleeve being cut in the process, the intruder, turning its attention to Rukia, uses the same technique on her, but Ichigo prevents the intruder from hurting her.
Realizing the light is more than blinding them, Rukia tells Ichigo to be careful. Despite this, he is caught once more in the technique, forcing Rukia to, coming to his aid, engage the intruder herself. Telling Ichigo she believes they are dealing with an assassin and to not look into the light, Rukia is cut while fighting the assassin, and Ichigo takes over the battle. Closing his eyes, Ichigo, reflecting the light back at his opponent, swiftly disarms him. After explaining how he won, Ichigo demands to know why the assassin is after Rurichiyo, but the assassin begins to flee. When Ichigo uses Getsuga Tenshō to stop the fleeing assassin, the resulting explosion kills the assassin.
Ichigo and his friends discover the reason for their mysterious behavior: Rurichiyo is the heiress of one of the most powerful noble clans in the Soul Society, the Kasumiōji Clan, the highest noble family which is not one of the Four Great Noble Clans. They explain how Gyōkaku Kumoi, one of the clan's vassals, has been behind several recent assassination attempts on Rurichiyo's life in a bid to become leader of the clan. For this reason, Kenryū and Enryū, having taken Rurichiyo to the World of the Living in order to protect her from assassination, request Ichigo's aid in this.