If you mean the individual chapter, it features Tsukasa and Tsukushi getting stuck in an elevator, forcing them to spend uncomfortable alone time together.
While on a trip, news breaks that Shizuka (Rui’s first love) is being courted by a French politician and may marry him. Rui is devastated and tells Tsukushi that "it's over". In a moment of vulnerability on the beach, Rui kisses Tsukushi, which is witnessed by Tsukasa (Domyoji). Hana Yori Dango Episode 21
The anime version consists of 51 episodes, and Episode 21 marks a critical turning point in the F4 dynamic. "Night of Betrayal". If you mean the individual chapter, it features