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