Stephon Marbury was, in his words, stressed and depressed.

He was on a plane headed to China, with no idea what was in store for his once-promising career. He had flamed out in the NBA, fallen well short of expectations playing for the hometown team he grew up rooting for, still not whole after losing his father, Don.

But when the plane landed in this foreign land, Marbury looked out the window and saw thousands of people waiting for the aircraft. He figured there was someone else important on board. No, they were there for him.

“I couldn’t stop smiling,” Marbury told The Post in a phone interview. “I was like, ‘There’s no way.’ I went from being told I couldn’t play basketball anymore, my game is done, I’m over with, to a place where people were smiling that I was coming to play basketball there. It kind of felt foreign.”

It started a memorable second act of his career for the Coney Island native, one that saw him win three Chinese Basketball Association (CBA) titles with the Beijing Ducks and later become a coach in the CBA. Unlike other American stars, Marbury made China a second home. He arrived in 2010 and hasn’t left. He became such a popular figure in Beijing, there was a bronze statue erected of him, a postage stamp honoring him and a museum created dubbed The House of Marbury Museum. There was even a musical loosely based on his life, starring him.

“You talk about somebody lifting your spirits. I can’t even explain it,” he said. “I’m treated with love.”

Stephon Marbury
Stephon Marbury1091, SLAM, RTG Features

This past decade of his life is featured in the new documentary, “A Kid from Coney Island,” that is set to premiere at Brooklyn Academy of Music on Thursday night before being released in select theaters on March 10. Developed by producers Nina Yang Bongiovi, Forest Whitaker and Jason Samuels, it traces Marbury’s journey, beginning as a basketball phenom growing up in the Surfside housing projects in Brooklyn, hoping to reach the NBA, a dream that has eluded other talented basketball-playing members of his family. It follows Marbury as a high school star at Lincoln to Georgia Tech in the ACC and his turbulent career in the NBA, his departure from the Timberwolves and his acrimonious time with the Knicks and battles with coach Larry Brown.

This is all accompanied by commentary from fellow players, media personalities, family members and friends. It isn’t until Marbury gets to China that we hear from him. That is done by design.

“A lot of people don’t know what his story was. A lot of people don’t know he went to China,” co-director Coodie Simmons said. “They just know his downfall and he left the NBA. We thought this can be a great departure from the [part about] the NBA, to finally see him, and hear him in China, and give the story of those moments of triumph.”

Marbury, 43, explains the decision. It wasn’t about basketball. It was about his mental health and getting a fresh start. It allowed him to focus, and his game took off. In his introductory press conference, the two-time NBA All-Star answered every question the Chinese media had, about his clashes with coaches in America, his off-and-on struggles on the court, the YouTube video he posted when he looked to be having a nervous breakdown. At the end, he asked them to judge him based on what he does in China.

The film takes you through Marbury’s career there, his three titles, and his emotional final game, the adoring fans, his teammates throwing him in the air after one of the championships. He gives the viewer a tour of his museum.

“It’s like going to another world,” he said during the documentary.

But it doesn’t end there. Marbury comes back to Coney Island, where it all started, taking us on a tour of where basketball began for him, the housing projects where he spent his childhood, and back to a local barber shop. There, Marbury speaks to a young kid about his accomplishments in China. The child, who gets a haircut like Marbury did when he was his age — with a middle part — wants to play in the NBA like Marbury did. Marbury tells him he doesn’t have to play basketball. He could be president one day. And that’s where it ends, coming full circle.

“It was pretty amazing. I got to see myself in a way that was told correctly,” he said. “It’s not perfect. There’s ups and downs, trials, tribulations that have gone on. But there’s also points in there that shows the rise.”

In watching the film, Marbury said he doesn’t regret any of the decisions he made while in the NBA, because he knows where it led him. He doesn’t know if this will change anyone’s opinion of him. That’s not why he agreed to make the documentary. It was to tell his story.

“There’s no new Stephon Marbury. There’s an evolved Stephon Marbury from experience,” he said. “You’re not the same you were five years ago. You learn, you grow.”