Because Hana Yori Dango (Boys Over Flowers) has several famous adaptations, Episode 21 refers to different pivotal moments depending on the version you are watching.
In this version, Episode 21 is a heavy-hitter for the "soulmate" storylines and the lead-up to the finale. Hana Yori Dango Episode 21
If you mean the individual chapter, it features Tsukasa and Tsukushi getting stuck in an elevator, forcing them to spend uncomfortable alone time together. Because Hana Yori Dango (Boys Over Flowers) has
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). While on a trip, news breaks that Shizuka
Jan Di and Jae Kyung (Jun Pyo’s fiancée) have a heart-to-heart about their feelings for Jun Pyo. Jae Kyung is preparing to walk down the aisle, but tension peaks as it becomes clear the groom isn't ready.
If you are looking for the manga content, deals with the aftermath of Kaede Domyoji discovering Tsukushi living in her home.
The anime version consists of 51 episodes, and Episode 21 marks a critical turning point in the F4 dynamic. "Night of Betrayal".