Which episode is Darcy and Kathy?

Darcy and Kathy are two characters in the fourth season of Underwater Column, in which Darcy is one of the protagonists and Kathy is Darcy's sister in 15. In the plot, Kathy came to visit the small column at the bottom of the sea. She is a girl who loves reading detective stories. In the play, she clapped her hands with the observation method in the novel.