Paul McCartney's first original orchestral score for ballet will debut at the New York City Ballet's fall gala on Sept. 22.

"I am always interested in new directions that I haven't worked in before," McCartney said in a posting on his website Thursday.

The former Beatle says the ballet, titled  Ocean's Kingdom, is a "romantic story" in which the daughter of the ocean king falls in love with the brother of the earth king.

"You'll have to see whether the couple make it," the 68-year-old told the New York Times. "There's all sort of troubles along the way."

McCartney created the score and composer John Wilson is helping with the final orchestration, according to the Times.

The New York troupe's ballet master, Peter Martins, will handle the choreography.

"I became very excited about the idea" after meeting Martins in 2010, McCartney said of the new venture in his web posting.

"What was interesting was writing music that meant something expressively rather than just writing a song. Trying to write something that expressed an emotion — so you have fear, love, anger, sadness to play with and I found that exciting and challenging."

The ballet, expected to run between 45 and 50 minutes and feature a cast of up to 45, will likely be performed in London after its New York City dates.

Prior classical compositions

McCartney has written orchestral scores before: his first was 1991's Liverpool Oratorio to commemorate the Royal Liverpool Philharmonic's 150th anniversary.

The most recent was Ecce Cor Meum (Behold My Heart ), released in fall 2006 and inspired by his first wife, Linda, who died of breast cancer.

In June, he told the BBC that he had been commissioned to write the music for a ballet, but wouldn't say at the time for whom he was writing the music and when it would debut.