Killer And Immortal TargetAssassin-chan and Immortal Target ; Koroshiya-chan to Shinanai Target ; 殺し屋ちゃんと死なないターゲット

How do you kill a target that can't die? Even worse, how can you possibly kill said target if they give you food, clothes and a hot shower? Will Miss Killer be able to kill the mafia boss Leonard Amaretto? On top of that... will she be willing to, even when the old man asks her to do so after receiving the kindness he dispenses her?