Long before Francis Ngannou could parade his leather-and-gold Ultimate Fighting Championship belt above swaths of dancing and shouting fans in his native Cameroon, he skulked out of a classroom there, too ashamed to see his classmates.

He was 13 years old, and a teacher had just kicked Ngannou out of the room because he lacked a pencil and other required school supplies. In that moment, as he often did to cope, Ngannou entered the “virtual world” he created for himself.

In that fantasy, his parents were still married and had not divorced when he was 6 years old. His family was well fed, and he had never needed to work grueling hours in sand mines. He didn’t have to deal with adult levels of stress. He was still a kid.

Ngannou, now 35, flashed back to that moment to help appreciate a more recent, happier memory: his visit to Cameroon last spring after he won the championship in the U.F.C.’s biggest weight class. He rode around the streets of his home country in the back of a pickup truck while being feted as a national hero, the crowning achievement of a nomadic, chaotic life in which he has both experienced spurts of homelessness and ascended to the height of a sport he is still learning.

“I wanted to prove I could do something on my own,” Ngannou said in a video interview in December. “The whole country was standing by my side, but it felt like the whole continent.”

On Saturday night, Ngannou (16-3-0) hopes that the resolve he has built over the last two decades can propel him to victory again when he defends his belt at U.F.C. 270 against the interim champion, Ciryl Gane, who briefly trained with Ngannou. Gane, who is undefeated with 10 wins, is a slight favorite among oddsmakers.

Each fight in a fighter’s career is important, especially a champion’s first title defense. And for Ngannou, this bout carries the additional weight of contract talks — he has argued that he is underpaid under his current deal with the U.F.C.— which have led to bickering, disagreements and uncertainty between the fighter and the promotional company.

Win or lose, this fight could be his last with the U.F.C.

An imposing physique belies a quiet personality.

In mixed martial arts circles, Ngannou is referred to colloquially as man on the planet. Even before he knocked out Stipe Miocic to claim the heavyweight championship, the U.F.C. used its own measurements to brag about his punching power with the hope of attracting fans.

Dana White, the U.F.C. president, said that being punched by Ngannou was more damaging than being hit with a 12-pound sledgehammer. Ngannou stands 6-foot-4, with chiseled muscles and protruding veins on his skin that are the size of small, slender snakes.

But those close to him say that behind his threatening physique is a quiet man who approaches his relationships with caution.

“He’s a big, ominous figure that knocks people out, but at the end of the day, he has a huge heart,” said Eric Nicksick, Ngannou’s trainer and the head coach at the Xtreme Couture gym in Las Vegas.

Ngannou’s personality, unlike the braggadocious personas usually assumed by prizefighters to persuade fans to spend $70 or more on a pay-per-view spectacle, was forged by the problems he faced early in life.

He was born in Batie, Cameroon, a town of about 11,000 people in the western part of the country that is roughly 130 miles inland from the Gulf of Guinea. On average, Cameroonians live on about $1,500 per year.

After his parents divorced, Ngannou shuffled between family members’ homes. He often observed how healthy an aunt and uncle’s relationship was and craved a stable environment. But that rarely came.

At 9 years old, he began working in sand quarries with his older brother, spending the time when he was not in school shoveling sand into trucks and completing other laborious tasks. While the pay helped his family, it still wasn’t enough.

“We couldn’t really enjoy our meals because we would be wondering where the next one would come from,” Ngannou said.

His father was known as a street fighter, and even though Ngannou did not like the dishonorable reputation that carried, he soon developed a love for boxing.

And so when his teacher dismissed him from the classroom, he determined that he would no longer accept living in poverty, viewing a professional fighting career as his escape.

A long, dangerous journey north had stops in Morocco, Spain and France.

At 22, he left Batie to move to Douala, the country’s largest city, to find a gym. As he began his training, he juggled odd jobs, like driving a taxi and unloading trucks.

“I was just surviving,” Ngannou said. “Everyone thought I was crazy, but I wanted to do something and have a purpose.”

As his boxing skills increased, he sensed that more chances to grow lay beyond Africa’s borders. “I can’t stay in Cameroon and expect a miracle,” he recalled thinking.

He decided to go to Morocco with the eventual goal of reaching Spain. For 14 months, he traveled by train and car — sometimes with the help of smugglers. He stayed in Morocco for one year, attempting — and failing — to reach Spain in different ways. He settled on the risky method of traversing the Strait of Gibraltar with other immigrants using an inflatable boat.

The American Red Cross intercepted the craft midway, though, and brought him to Spain, where he was detained for two months for illegal entry.

After his release, he said, he wanted to go to Germany, where he felt the avenues for boxing were better — but he boarded the wrong train and arrived in Paris. Exhausted, he quickly accepted that France could be a good option, too.

“At the end of the day, all I needed was an opportunity,” Ngannou said.

Ngannou slept in a parking lot during his first night in Paris, and the next morning he began searching for gyms, asking strangers for recommendations. He soon found both a gym and a coach. For two months, he lived on the streets, relying on services like public showers and lockers until someone he met at the gym temporarily offered him the use of an apartment.

He later rented a room from someone’s home in a Paris suburb for 300 euros a month, which is equivalent to about $340. All he had, he said, was a mattress the person had lent him, which he placed in a corner, and a duffel bag with his gear.

Though he had learned about mixed martial arts at the gym he had joined, he had not initially been interested. But when he heard that the sport could potentially offer more money than boxing, he began exploring it. He soon met Fernand Lopez, the head coach at MMA Factory, one of the largest gyms in France. The two bonded, and Ngannou started taking mixed martial arts seriously and winning fights around France.

He then joined the U.F.C. in 2015, and his career path — at least for the near and intermediate future — was set.

“I wasn’t doing M.M.A. just for fun anymore,” Ngannou said.

Ngannou’s punching power made him a star.

Ngannou’s ascent within the U.F.C. happened quickly.

He won his first six fights — all from finishes — and capped the stretch with a devastating left-handed uppercut of Alistair Overeem.

“You don’t see people with just that one punch, raw power,” White said in an interview. “When he hits people, lights go off and that’s it. It’s just vicious.”

Watching the Overeem fight, White knew Ngannou could challenge Miocic for the championship.

The buildup for U.F.C. title fights and pay-per-view main events differs from that of other contests. Athletes often say the extra media attention and other obligations are time-consuming, and those who are not used to the spotlight say they can be draining.

Ngannou said that the process was more difficult than he expected, and that, by the time the fight arrived, he was tired. It showed in his performance as Miocic — who had already defended the belt twice — thoroughly outclassed Ngannou, winning via unanimous decision.

The January 2018 bout exposed holes in Ngannou’s wrestling, which he said motivated him to improve. But he lost his next fight five months later via decision to Derrick Lewis, a contest in which he seemed timid and unaggressive. He won his next fight, but his relationship with Lopez soured, and he soon left France to move to Las Vegas permanently.

There, he rebuilt himself into a title contender, winning his next three fights, two of them with Nicksick as his new trainer.

Nicksick learned quickly that Ngannou was a reserved person, but through training sessions and the time they spent together, the two bonded and Ngannou became close with Nicksick’s family.

“He’s not going to let a lot of people in,” Nicksick said. “I respected that, and I feel like that’s why we’re at where we’re at. It’s been earned through sweat equity and a lot of time on the mat.”

Ngannou and Miocic headlined U.F.C. 260 in March of last year.

Ngannou’s improvement was evident, especially on defense. In the second round, the fighters engaged in a punching exchange, and Ngannou tapped Miocic on the chin with his left hand and knocked him unconscious. Afterward, unlike many challengers who unleash bursts of emotion after a championship victory, Ngannou calmly walked to his corner, pointed skyward and then raised his hands in a prayer motion.

By winning the belt, he became the third simultaneous African-born champion, joining middleweight Israel Adesanya and welterweight Kamaru Usman, who were both born in Nigeria. In the locker room afterward, he said he felt the weight of that achievement, and that he “knew they were waiting on me to get it done.”

It’s unclear whether Ngannou and the U.F.C. will extend their contract.

The championship brought more fame. But Ngannou’s short reign has been dominated by his push for bigger pay days.

The U.F.C. wanted Ngannou to fight Lewis just three months after the battle with Miocic. Ngannou and his agent, Marquel Martin, did not accept that rapid turnaround, and the U.F.C. created an interim title between Lewis and Gane instead. The move drew criticism from some fans and observers. White said that he stands by the decision as one that was necessary to keep the division moving. But it was still frustrating, Martin said.

“Respect is No. 1, especially for Francis,” Martin said. “That’s just the way he’s built.”

Ngannou’s fight with Gane is the last bout on his current contract, and Ngannou’s camp and the U.F.C. have yet to agree on a new deal.

The spat has seeped into the public — White and Martin have traded barbs on social media, and Ngannou has said he is prepared to test free agency, though he wishes to stay in the company.

The U.F.C. has the option to extend a contract if a fighter wins a championship or defends it — a standard provision that has become a target of some fighters and others critical of the U.F.C.’s pay practices. But White has said Ngannou is welcome to leave regardless of Saturday’s result.

The two main points of contention are money — in an interview with ESPN, Ngannou said he will no longer fight for “$500,000 or $600,000” — and more freedom. Ngannou would still like to box, and he pointed to the 2017 crossover event between the boxer Floyd Mayweather Jr. and the U.F.C. star Conor McGregor as a template. Tyson Fury, the World Boxing Council heavyweight champion, has also teased on Twitter that he would be interested in a bout.

White said the McGregor-Mayweather fight was an outlier that wouldn’t make sense for Ngannou, who is much less famous than McGregor.

This month, White and Ngannou, in an unplanned coincidence, ate dinner at the same time at Delilah, an upscale restaurant at the Wynn Las Vegas resort and casino, where they respectfully discussed their frustrations. Both are hopeful they can come to a resolution, but it remains unclear how that might happen.

“At the end of the day, I want him to want to be here,” White said. “We’re building the sport, building the athletes and the fun in that is doing it together. If you really want to be somewhere else, then you probably should be.”

It’s a situation that will follow Ngannou during and after the fight. But as he has done throughout his life, it is another problem he feels will be resolved by following his emotions and betting on himself.

“We have to stand up for ourselves,” Ngannou said. “This is something we can work through, if they’re willing to make it happen.”