Last month, three teenagers from Brooklyn got on an F train. Their goal was to help their basketball team win. However, their journey took an unexpected turn when they ended up capturing a dangerous criminal suspect.

Christos Strieder and his pals, Kingson and Navid, made the fateful 911 call on Dec. 22 that landed sick firebug Sebastian Zapeta-Calil behind bars for allegedly torching a sleeping homeless woman to death on the same train line earlier that day.

The 14-year-olds had only learned about the heinous crime a mere five minutes earlier as a warning from one of their mothers — and they were shocked to actually run into him out of the millions of people in the transit system.

“It was that big of a coincidence to the point where I felt like I was dreaming — like it wasn’t even real. Things happen in the world, but you don’t actually feel like it’s going to happen to you,” Kingson recalled in an exclusive interview with The Post.

“It just felt really surreal to me. I didn’t really think anything of it at first. But then once I actually got home and started thinking about it, everything really started to set in that that really happened.”

The friends, all freshmen at Millennium Brooklyn High School, had hopped on the F train at 7th Avenue that Sunday as they headed to a junior varsity basketball game in Queens.

Having just read the article sent by Navid’s mother about the tragic arson murder, the picture of the suspected firebug was fresh in their mind when Christos noticed an otherwise non-descript man sleeping in the train car.

“We walked onto the train and we were joking like, ‘He’s going to be on this train’ and then we walk on and I see him,” said Christos, who lives in Kensington.

“They didn’t believe me at first because they thought I was joking. But then after a couple of minutes, they also looked at him and then they realized, ‘okay, that’s actually him’ . . . The tone got a little bit more serious.”

Zapeta-Calil was wearing the same black hoodie and gray sweatshirt he donned when he allegedly ignited Debbie Kawam earlier that day. Much like his victim, the sadist was also sleeping as he rode the rails.

Somehow, no other straphangers on the crowded F train noticed the accused firebug — despite news of the horrific crime ripping through the city and stirring a frenzy.

The teens realized it was up to them to turn the Guatemalan migrant over to police.

Kingson, of Bensonhurst, who like Navid asked his last name not be used, was the one to discretely call 911 without drawing the attention of the other commuters.

“There was nothing to lose,” he said.

The trio remained in the car to keep an eye on the allegedly deranged man. Fortunately, Zapeta-Calil didn’t wake up before the NYPD stopped the train at 34th Street-Herald Square.

“On the loudspeaker, they said they’re going to go through the doors one by one and open them because there’s some mechanical issue,” Christos recalled.

“They actually searched our car one time and they didn’t notice him! Then the second time they went, they saw him. There was a bunch of police officers who arrested him, got him out. And that’s when we told the police officers that it was us who called.”

The officers sang the eagle-eyed boys’ praises for making the fateful call and asked the trio to follow them to the station for follow-up questions, but the friends declined.

The whole ordeal had already made them late for their basketball game.

“We really value basketball as something that we love,” said Kingson, adding that the three made it in time to help bring their team to victory.

“We won. It was a pretty good game.”

The friends were hailed as heroes by their teammates and parents, and Kingson was even awarded a City Council citation from Councilmember Susan Zhuang (D – Bay Ridge) earlier this month for dialing 911.

Despite their incredible experience, neither Kingson, Christos nor Navid has any new apprehensions about riding public transit, although they admit they learned a lesson about being attentive in the underground.

“I always make sure that I look for my surroundings. This experience, it just shows how the subway is like a dangerous place, so make sure to be on the lookout because you just have to take the train sometimes because Uber is expensive,” Navid, of South Slope, told The Post.

“I’ve just gone on with my life. It wasn’t like a big thing I’m worried about.”

Zapeta-Calil, 33, allegedly told cops he was so drunk when he torched Kawam days before Christmas that he doesn’t even remember the appalling attack.

The migrant is accused of flicking a lighter on Kawam, a 57-year-old from Toms River, NJ as she slept aboard an F train while it was docked at the Stillwell Avenue-Coney Island station.

He then sat and watched her burn alive — and even used his jacket to fan the flames, prosecutors allege.

Zapeta-Calil has pleaded not guilty to murder and arson charges and is due back in court in March.