We've long spoken about how 'The Simpsons' has gone well past its sell by date. Still, those behind the sitcom are determined to keep things fresh.

Last weekend, Comic-Con took place and during it, season 33 of 'The Simpsons' was discussed.

The animated TV series is set to have its very first all-musical episode, titled 'The Star of the Backstage'.

It'll be the new season's premiere episode and airs on September 26th.

Executive producer Matt Selman describes it to EW as "the most musical episode we've ever done."

He added: "Almost wall-to-wall music. It's like a Broadway musical of an episode with all original songs."

Kristen Bell of 'The Good Place' and 'Frozen' fame will feature as Marge Simpson's singing voice.

"We all love Marge's voice [Julie Kavner]," said Selman, "but this is the singing voice that's different, let's just say."

He explained: "The only place Marge sang beautifully was in her head, so she has a magical inside singing voice that only we can hear.

"When she sings, it comes out beautiful, like Kristen Bell."

The new 'Simpsons' episode's plot is as follows: "Marge has amazing memories of being the stage manager of her high school musical, Y2K: The Millennium Bug, and decides to restage it with everyone 20 years later for one last show.

"But when her old high school nemesis comes to town, she realizes that her high school memories aren't what she thought they were."

The episode was written by Elisabeth Kiernan Averick ('Crazy Ex-Girlfriend').

In other 'Simpsons' news from Comic-Con, this year's Treehouse of Horror special will feature five segments instead of the usual three.

In the autumn, there's going to be a two-part episode called 'A Serious Flanders".

It serves as "epic love letter to the show Fargo, prestige crime dramas, and the world of streaming television."

Brian Cox, Cristin Milioti, and Timothy Olyphant feature in the episode.