Watch Tonight's 300th 'Glee' Musical Performance: An Adele Mash-Up (VIDEO)

"Glee" hasn't even been on the air for two and a half seasons yet, but the show is nonetheless celebrating a huge milestone this coming Tuesday: the episode will feature the 300th musical performance on "Glee."

That's pretty impressive considering this will only be the show's 50th episode, after 22 episodes in the last two seasons and nine so far in the current season. That averages out to six musical numbers an episode. It's kind of amazing that they managed to fit any plot in there at all, with all the singing and dancing going on.

The song will be a mash-up of two Adele songs: "Rumor Has It" and "Someone Like You." Funny enough, this big milestone number isn't sung by New Directions, or even any of its current members.

Rather, Santana (Naya Rivera) and Mercedes (Amber Riley) will handle the duties with their new all-girl group, the Troubletones.

The episode is titled "Mash Off" and will feature a number of other musical mashups.

Watch the video of the 300th "Glee" musical performance below: