"The Flash" Season 3 episode 17 "duet" will have a cross-over with "Supergirl". Supergirl and her friends turn to Barry and Team Flash for help when "Glee" alum Darren Criss attacks Kara. 

"Glee" actor Darren Criss will be appearing in "The Flash" Season 3. According to Carter Matt, the 30-year-old American actor and singer has landed the guest role of Music Meister.

Music Meister is a villain who has a silly surface but possesses a dark and sinister personality.

As for the official plot synopsis of episode 17, Spoilers Guide notes, "Barry Allen (Grant Gustin) and team are surprised when Lar 'Mon-El' Gand (guest star Chris Wood) and Hank Henshaw (guest star David Harewood) arrive on their Earth carrying a comatose Kara 'Supergirl' Zor-El (guest star Melissa Benoist) who was whammied by the Music Meister (guest star Darren Criss). Unable to wake her up, they turn to Team Flash to save her.

However, the Music Meister surprises The Flash and puts him in a similar coma, one that Team Flash can't cure. Kara and Barry wake up without their powers in an alternate reality where life is like a musical and the only way to escape is by following the script, complete with singing and dancing, to the end."

Moreover, the previous episode showed how Barry managed to save Wally West (Keiynan Lonsdale) from being trapped at the speed force. Also, by the end of the episode, Barry realized that things between him and Iris (Candice Patton) just could not work out. It seems like he was not able to move on yet from the pain brought by the death of his parents. In addition, team Flash uncovers a reality that Savitar still has weakness under his armor.

Viewers can remember how Savitar calls himself a god. Unfortunately, he was not all powerful than who people think he is. Up to now, his true identity is still a mystery. As Inverse pointed out, fans can dig more about "Blue Flash" arcs in the comics. Furthermore, the said outlet reports the possibility that Savitar is the Barry Allen from the future.

"The Flash" Season 3 episode 17 is titled as "Duet". The upcoming episode is scheduled to premiere on Tuesday, March 21, 2017, at 8:00 p.m. EST first and exclusive on The CW.