Hana Yori Dango Episode 21 -

If you are looking for the manga content, deals with the aftermath of Kaede Domyoji discovering Tsukushi living in her home.

If you mean the individual chapter, it features Tsukasa and Tsukushi getting stuck in an elevator, forcing them to spend uncomfortable alone time together. Hana Yori Dango Episode 21

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. If you are looking for the manga content,

To protect her friends from her mother-in-law's wrath, Tsukushi agrees to stop seeing Tsukasa. Tsukushi agrees to stop seeing Tsukasa.