In ‘Crazy Ex-Girlfriend,’ Rachel Bloom stalks and bursts into song

Driven young attorney Rebecca Bunch is on a career hot streak, about to add “junior partner� to her résumé and pave her professional path as a Manhattan legal eagle.

But she chucks it all after bumping into an old summer-camp flame and seeing an oddly haunting butter commercial that asks, “When was the last time you were truly happy?� In short order, it’s bye, bye, white-shoe law firm, hello, West Covina.

That’s crazy, right?

Rebecca, played by YouTube star Rachel Bloom, serves as the off-balance center of the high-concept dramedy “Crazy Ex-Girlfriend,� premiering Oct. 12 on the CW. Paired with critical darling “Jane the Virgin,� the series is the network’s biggest and in fact only bet for new programming this fall.

Full coverage: Fall TV guide 2015

If the setup about an anxious, delusional gal who pulls up stakes to chase a guy named Josh Chan across the country doesn’t seem wacky enough, there’s more: It’s a musical.

At least a couple of times during each hour, Bloom and her fellow cast mates will burst into song to give voice to her inner monologue. And they won’t be doing covers of top tunes, à la “Glee,� but original numbers in a wide range of genres.

In the pilot alone, there’s a big Broadway-esque tribute to the quaint burg of West Covina — two hours from the beach, four with traffic! — complete with a Busby Berkeley-style dance routine at an open-air mall. Bloom, who’s also an executive producer on the show, ends up aloft on a giant fake pretzel for the showstopper moment.

Bloom, a Manhattan Beach native who describes herself as “a dark person who loves Disneyland,� said the series represents “the flip side of a rom-com.�

“We’re exploring what it’s like to be crazy in love,� Bloom said recently from the set of the show’s North Hollywood studio. “She’s not an ‘SNL’ character but a very intelligent person who has the veneer of sanity but really is miserable, grasping at anything for fulfillment. She somehow thinks that Josh Chan is the solution to all her problems.�

“Crazy,� then, isn’t meant as a pejorative term or a dig at people with mental health issues, said executive producer-creator Aline Brosh McKenna, whose film writing credits include “The Devil Wears Prada� and “27 Dresses.�

“It’s in the vernacular for a reason: We’ve all either been a crazy ex or had one,� McKenna said from the sidelines of the “Girlfriend� set. “We’re getting inside what it’s like to be that person. She makes drastic, ill-advised decisions and then has to work through the ramifications.�

The series was developed initially as a half-hour comedy for Showtime, with the premium channel interested in the hybrid nature of the show and willing to take a chance on an Internet star. Bloom, who’s since been named to a number of talent-to-watch lists, may be best known for her musical comedy videos with names like “Historically Accurate Disney Princess Song,� and “You Can Touch My Boobies� that get millions of views.

The pay cable channel, however, passed on “Girlfriend� this year. McKenna and Bloom shopped it elsewhere, a process they described as “one of the stages of grief,� and had nearly resigned themselves to rejection.

But McKenna, a super fan of “Jane the Virgin,� thought the CW would be a great fit for the whimsical, female-centric story. CW executives agreed and in April asked the creators to come up with an hourlong broadcast-friendly version.

Working quickly but thinking the series would be a midseason contender, McKenna and Bloom stripped out the swear words and toned down the sexy stuff. There was no nudity anyway, they said, just a bit of raunchy humor. With an additional 13 minutes, they added and fleshed out supporting characters and expanded the show.

They were surprised to learn that the CW wanted the dram-com for the fall season and that the network brass “didn’t try to sanitize the darkness,� Bloom said. “That floored me.�

The series is definitely sun soaked — some of it is filmed in West Covina — but doesn’t skimp on Rebecca’s struggles. She’s moved 3,000 miles to become a stalker, after all, and she sticks out like a sore thumb in a new job for which she’s obviously overqualified. And soon after she arrives, she learns that Josh Chan (Vincent Rodriguez III) already has a girlfriend.

West Covina gets star billing in what McKenna calls a homage to the Los Angeles ‘burb, with Rebecca being that big-city gal ready to dive into small-town life. “She’s a fish out of water here, but her embrace of it is wholehearted,� she said.

There will be upward of 25 original songs in different genres during the season’s 13 episodes. Expect a Bollywood number, Bloom said, along with pop, rock and slow-jam R&B, to name a few.

It’s no accident, then, that the cast is populated with triple threats, including Broadway veteran Donna Lynne Champlin (“Billy Elliott,� “Sweeney Todd�) as Paula, a co-worker who becomes Rebecca’s enabler, wingwoman and surrogate mom.

The network that’s home to “Arrow,� “The Flash� and “The Vampire Diaries� deliberately added only one new show for the fall launch, “so that people understood we were stable,� CW President Mark Pedowitz said during the recent Television Critics Assn. press tour.

That alone is considered a risk, and building its first comedy-centric night with “Girlfriend� and “Jane the Virgin� on Mondays is another departure for the drama-heavy channel.

But Pedowitz has faith in “Girlfriend,� which caught his eye because of its quirky perspective and musical elements. “We think we’re taking a swing at something that no one else has on the air,� he said.