Episode 21 is called "Wonder-Ful". It's a Stevie Wonder tribute episode.
Kurt returns to Lima for Burt's medical appointment. Mercedes and Mike return to help New Directions prepares for Regionals, and in New York City Cassandra makes an unexpected gesture.
Rachel, Kurt, Cassandra, Burt, Carole, Marley, Ryder, Jake, Kitty, Unique, Blaine, Sam, Tina, Artie, Mike, Mercedes and Will are all in the episode.
There'll be 6 solos and 1 duet. Artie will have one, Kurt will have one, Mercedes will have one.