The leaders on the Kansas City Chiefs’ sideline repeated the word over and over even as their fate grew more and more dire: Believe. They trailed the Tampa Bay Buccaneers by three touchdowns late in the third quarter. They had committed penalties, dropped passes and shanked punts all through the Super Bowl against the greatest quarterback ever, an opponent who leaves no mistake unpunished. But they had Patrick Mahomes, and so they had reason to believe.

For the first four years of his charmed career, Mahomes won a Super Bowl, became the face of the NFL, started a business empire, transformed Kansas City’s image of itself, urged Americans to vote and launched a presumed dynasty. He also made the Chiefs impervious to humiliation. He almost never lost — and when he did, the final seconds determined the outcome.

Mahomes is still a quarterback worth believing in, and the Chiefs may still stack Lombardi trophies. But no longer can he claim to be unacquainted with the wrong end of an NFL butt-kicking. The Chiefs suffered the biggest defeat of Mahomes’s young career in perhaps his biggest game, a 31-9 loss that delayed, or possibly derailed, Kansas City’s burgeoning dynasty and made the task of Mahomes surpassing Tom Brady in the NFL annals an even more astronomical mission.

The Chiefs’ startling demise came swiftly and thoroughly. They did not score a touchdown for the first time since Mahomes and Coach Andy Reid joined forces and revolutionized NFL offense. They did not score double-digit points for the first time with Mahomes behind center and failed to reach 20 for the third time in his 54 career starts. Mahomes had never lost by more than eight. He had lost nine times in his career, including playoff games, by a total of 44 points. In one night, the Buccaneers dusted him by half that total.

“I didn’t see it coming at all,” Reid said. “I thought we were going to come in and we were going to play these guys just like we’ve been playing teams. And it didn’t happen that way. They did a nice job. Give them credit. I didn’t anticipate this coming at all.”

Aside from a meaningless Week 17 loss when they sat almost every key player, the Chiefs entered Sunday night with a 25-1 record in the past 15 months. Along the way, Mahomes frequently conjured magic. Reid’s play-calling aggression and Mahomes’s ballistic arm strength allow the Chiefs to play from behind in comfort. The Chiefs turned three double-digit deficits into playoff victories last season and transformed a 9-0 hole in this year’s AFC championship game into a romp over the Buffalo Bills.

Mahomes uses a different kind of competitive math than any other quarterback. He said he did not realize the Chiefs would lose until he threw an interception in the end zone with 1:40 left in the fourth quarter.

“You knew it was pretty much over then because we had no timeouts,” Mahomes said.

Faced with a postgame reality that everyone else had processed far earlier, Mahomes reckoned with a new professional disappointment. He had lost the AFC championship game in his first try, but he had never lost at a level he had previously conquered. He played on outmanned teams at Texas Tech, but he has not processed a blowout loss in the NFL. His point of reference was his father, a longtime major league reliever.

“My dad lost in the World Series in his career, and he continued to battle and continued to be who he was,” Mahomes said. “It hurts right now. It hurts a lot. But we’re going to continue to get better. We have a young group of guys who have had a lot of success and learned from that. But we’ve had a few failures. We have to learn from that. We can’t let this define us. We have to get better going into next year.”

The Chiefs’ problems in the Super Bowl began early in the week, in tragic fashion that is not yet fully understood. Britt Reid, Andy’s son and the Chiefs’ outside linebackers coach, was involved in a car crash that left two children injured, including a 5-year-old in critical condition. A Kansas City television station reported that, according to a search warrant, a police officer said “a moderate odor of alcoholic beverages” was detectable and Britt Reid’s eyes were bloodshot. Britt Reid did not travel to the Super Bowl.

Andy Reid said his “heart bleeds” for the family affected, but “from a football standpoint, that wasn’t the problem.”

“It’s hard to put into words,” Mahomes said. “It didn’t take the air out of it. Guys were still ready to go. But it’s a very tragic situation. You know, you want to keep that in the back of your mind. You give prayers to the families that were involved, and especially the child that was involved. I don’t want to say it affected us on the field. They beat us. There’s no excuses on that. But you’re praying for those families.”

On the field, when the Chiefs did not self-combust, they were outmuscled and outsmarted. The Buccaneers throttled the Chiefs along both lines, and the designs of offensive coordinator Byron Leftwich and defensive coordinator Todd Bowles were a step ahead all night.

Bowles’s plan to stymie the Chiefs, after all seven teams with coaching vacancies passed on the former New York Jets head coach, ranks among the all-time Super Bowl coaching masterpieces. The Chiefs planned for the Buccaneers to play their usual style, mostly man coverages with a little zone mixed in and heavy blitzing.

Instead, Bowles adjusted to both his strengths and the Chiefs’ weaknesses. Bowles kept two safeties back deep in zone coverage all night, switching between cover-two and cover-four, to take away the speed of Tyreek Hill. He trusted blazing linebackers Devin White and Lavonte David to contain tight end Travis Kelce over the middle. He believed his front seven could stop running plays even with the safeties backed up. Bowles knew his pass rush could devour the Chiefs’ offensive line, which missed injured starting tackles Eric Fisher and Mitchell Schwartz, so he blitzed just five times.

Both scheme and personnel left Mahomes with little chance. He scurried for yardage and scrambled deep into his own backfield to keep plays alive, but the Buccaneers battered and chased him all night. Mahomes ran 497 yards before making throws or taking sacks, according to NFL Next Gen Stats, the most in one game since it started keeping track.

“Todd Bowles, he did his thing tonight,” Hill said. “They came out, and they just had a better game plan.”

Kelce said: “They made us dink and dunk, take what was there. They put a cap over top and didn’t let us get behind the defense, knowing how much speed we have. Sure enough, what that made us do is try and run the ball a little bit better, but their front seven is second to none in the league. It was frustrating. It was one of those where you felt like anything you did, they had an answer for.”

The Chiefs were co-conspirators in their own defeat. Faced with the quirk of playing in Tampa Bay’s home stadium, they looked like a road team. The first half was a procession of boneheaded errors. Kelce, typically an automatic catcher, dropped a crucial third-down pass deep in Kansas City territory, which led to rookie Tommy Townsend’s shanked punt. The Chiefs lined up offside on a field goal, which granted a first down and led immediately to Brady’s second touchdown pass to Rob Gronkowski. Defensive lineman Chris Jones got a 15-yard unnecessary roughness penalty for slapping antagonistic center Ryan Jensen. Chiefs cornerbacks played aggressively, and once officials flagged their handsy coverage, they failed to adjust.

“It was a bad day for us to kind of have a bad day,” Chiefs safety Tyrann Mathieu said. “Usually, we complement each other really well. You got to learn how to win in this league, and you got to learn how to lose. Losing is the hardest truth you have to learn.”

It was a lesson, too, for anyone who had taken the Chiefs’ oncoming reign for granted. The Chiefs have an innovative coach, an all-time quarterback who is only 25 and loads of offensive and defensive weapons. But Super Bowls can never be assumed. Brady and the New England Patriots won three Super Bowls in four years to start his career, and then a decade passed before he won another.

If Mahomes does get back to winning Super Bowls, he will start at a 7-1 deficit against Brady, the quarterback he will be measured against in any all-time conversation. A 6-2 edge would look and feel much different. Mahomes must now prove he can solve the next defensive coordinator with a rugged front seven who plays his safeties back in deep zone.

“When we joined together, we knew it wasn’t going to be always successful, and you weren’t going to be able to win 1,000 championships in a row,” Mahomes said. “We knew we were going to go through times like this.”

Maybe Mahomes knew, but nothing could have prepared an observer for what transpired Sunday night. Knowing Mahomes is at risk of being blown out and shut down is one thing. Seeing vivid proof on the sport’s grandest stage is another. Mahomes will get on with his career, starting Monday when, he said, he’ll consider surgery on his damaged toe. It is still easy to believe in him, but maybe not quite as easy as it once was.