The episode features Rachel, played by Lea Michele, singing "No One Is Alone" from Sondheim's musical Into the Woods.
The episode will also feature Rachel and Blaine, played by Darren Criss, joining forces on the Sondheim tune "Broadway Baby," from the musical Follies, according to TVLine.com.
"It's an anthemic Broadway song, we love Stephen Sondheim and we're Broadway theatre geeks that love singing this kind of music," Criss said in the interview.
"I sang this song when I was like eight years old. It was nice!" Michele added.
Check out the songs below:
"No One Is Alone."
Darren Criss and Lea Michele sing "Broadway Baby."
"Not While I'm Around."
"I'm Still Here."