Glow Glee: 'Glee' story and ratings keep hitting wrong notes

It’s almost irresistible to say “Glee” jumped the shark two weeks ago, when Shirley MacLaine sang Janis Joplin.

But alas, as even many remaining hard-core Gleeks must admit, this show started losing its once-dazzling luster many songs ago.

As “Glee” tees up its season-five finale Tuesday night (Fox, 8 o’clock), it has become a good illustration of a pervasive television problem: a show outliving its original fresh idea.

“Glee” is a particularly good example because it returns next fall for a sixth and final season.

Fox renewed it for seasons five and six more than a year ago, presumably before the network foresaw its plunge in popularity.

Last Tuesday “Glee” had 2.1 million viewers. That’s way down not only from the 12.45 million who watched the season two premiere, but from the 8.3 million viewers it averaged in season 5.

This past Tuesday it had fewer viewers than “The Deadliest Catch” on Discovery. Among young viewers, the ones networks really want to attract, “Glee” was tied with “Ink Master” on Spike.

So why did all these fans go?

Safe bet: They feel the show left them, not vice versa.

Which is not to say creator Ryan Murphy screwed up.

From the beginning “Glee” was different, starting with the fact it showcased musical numbers.

That made it a novelty act in the best sense of the term, and its near-instant success reflected the payoff for doing something fresh.

But if novelties can burn bright, they also can fade quickly. The fact “Glee” kept the flame strong through four seasons is the most impressive part of this picture.

Fox Darren Criss and Shirley MacLaine tackle a duet on ‘Glee.’

Murphy also compounded his own challenge by not keeping the characters in high school forever.

That was the right move for several reasons, including the fact the actors started in their 20s. It did mean he had to replace the students while giving the graduates new settings and stories.

It didn’t help that Corey Monteith died, eliminating his Finn Hudson character. But Finn had already illustrated Murphy’s dilemma, bouncing all over the show.

While some of the replacement students were engaging, like Melissa Benoist as Marley Rose, collectively they never matched the chemistry and freshness of the original cast, with Finn, Puckerman, Quinn, Brittany and the quirky others.

Moving Rachel (Lea Michele) and Kurt (Chris Colfer) to New York kept two of the most popular characters in the game, but tying them back to McKinley High proved awkward.

“Glee” soon felt like separate shows, and this season’s New York story played like a spinoff about the struggles of young performers on Broadway.

That’s how we got Shirley MacLaine and Darren Criss singing “Piece of My Heart.”

And why we’ve lost Mr. Shu (Matthew Morrison) and almost all the amusing McKinley High byplay that was so instrumental in the pace, rhythm and attitude of the original show.

Also, of course, a shrinking audience means a shrinking budget for a large ensemble cast.

“Glee” still has its moments and its songs. But it doesn’t seem accidental that it took every opportunity this season to revisit its past.

The upcoming final season could be amazing. But the theme song of “Glee” right now feels more like “Glory Days.”