The gang’s all here! Dennis, Dee, Charlie, Mac and Frank are taking over 111 Forum with No Refund Theatre’s “The Nightman Cometh” this Halloween weekend.

This musical is based on an episode of “It’s Always Sunny in Philadelphia,” in which the character Charlie writes a musical for him and his friends to perform in hopes of wooing the Waitress, his ever unrequited love. His absurd theatrical piece involves paying the troll toll to get into this boy’s hole, karate, and friendship for everyone.

This particular production of “The Nightman Cometh” combines the musical with elements from other episodes of the television series. Infamous bits such as the “D.E.N.N.I.S system” and “the gang forms a band” are weaved into the plot of the show. Another interesting element of this version of “The Nightman Cometh” is the improvised portions of the show.

“We were encouraged by our direction to add improvised bits to our characters,” said Jon Saperstein, who plays Dennis Reynolds. “The parts that we’ve created really feel like a part of the show.”

But don’t be discouraged from seeing “The Nightman Cometh” if you’ve never seen “It’s Always Sunny in Philadelphia” before. “The show is for everyone,” said Saperstein. “Even if you’ve never seen a single episode, you will be throughly entertained by this production.”

“The Nightman Cometh” opens tonight at 8 p.m. and runs till Saturday in 111 Forum. You can find more information on the musical’s Facebook event. Like all NRT shows, admission is free.