On a crisp October evening in Anaheim, Calif., the UFC heavyweight champion marched into a packed arena, shoved a police officer out of his way, and then promptly got his behind handed to him by a challenger who shrugged off a 20-pound weight difference to give him the most lopsided beating of his young career.

It was frantic. It was messy. It took just a shade over four minutes.

When it was over, Cain Velasquez was the new UFC heavyweight champion. Brock Lesnar was the big, bloody guy stumbling down the octagon steps with his face split open, his Viking beard stained crimson, staring in numb silence at the former pro wrestling colleague who stood there waiting for him.

Whether anybody knew it or not at the time, this was the beginning of something. Depending on your perspective, that something might have been an ending.

The date was October 23, 2010. For the main event of UFC 121, Zuffa executives had put together the biggest fight possible between the sport’s two top heavyweights. One was a stoic buzzsaw of a man who’d never been beaten. The other was a superstar pro wrestler who had jumped straight into the highest level of a new sport and now made headlines with his every move.

But just a year earlier all that was in jeopardy. After debuting in the UFC with a submission loss to Frank Mir in 2008, Lesnar had rebounded to win the UFC heavyweight title in just his fourth pro fight, beating a severely undersized Randy Couture at UFC 91.

That event would reportedly top 1 million pay-per-view buys, effectively minting Lesnar as the UFC’s newest box office star just as he laid claim to the belt that traditionally came with the title “baddest man on the planet.” Lesnar would defend it again at another million-buy event the following summer, getting his revenge on Mir at UFC 100.

But just as the UFC was angling for a showdown between Lesnar and Shane Carwin, the division’s other terrifying behemoth, illness struck the champion. While on a hunting trip in the Canadian wilderness, Lesnar had to be rushed to a nearby hospital (which he would later claim was so thoroughly incapable of treating him that it nearly caused his death at the hands of the Canadian healthcare system). The culprit was diverticulitis, a painful disorder that would eventually require surgery to remove a foot of Lesnar’s intestine.

By the summer of 2010, however, Lesnar appeared well enough to return to competition. He finally met Carwin, who had become the interim heavyweight champion, that July. After spending most of the first round being beaten to a bloody pulp, Lesnar survived to submit an exhausted Carwin early in the second.

With that, his comeback story seemed complete. Lesnar was officially the man once again. When the UFC booked him to fight Velasquez some three months later, the champ seemed to be feeling it, too.

Lesnar was never short on confidence, but he seemed to have some extra swagger when he showed up in Southern California that fall. At the pre-fight press conference inside the dizzying walls of the Walt Disney Concert Hall in downtown Los Angeles, the newly bearded Lesnar just chomped his gum and shook his head when asked whether Velasquez was his most dangerous challenger to date.

“I think they’re all the same,” Lesnar said.

The fans ate it up.

Velasquez, meanwhile, was the quiet favorite among many of his peers, even if he was the underdog with the bookmakers. Sure, he’d give up plenty of size against Lesnar, but he also had speed and technique and cardio. He was the rare heavyweight whose motor never seemed to slow, and it became such an omnipresent topic of conversation surrounding his fights that Lesnar seemed already sick of it a few days before the fight.

“That’s the only thing I ever hear about, is Cain Velasquez’s conditioning,” the champion complained.

But Velasquez had power, too. He’d punched his ticket to the title fight with a first-round knockout of former PRIDE heavyweight champion Antonio Rodrigo Nogueira some eight months prior. Before that, he ran through Ben Rothwell in a performance that proved just how effective his method of constant assault could be against a bigger, slower fighter.

At the weigh-ins the day before his big title shot, a grimly resolute Velasquez promised a war. When asked how he saw the fight going down, Lesnar was slightly more assertive.

“Brock Lesnar getting his hand raised,” he said. “That’s exactly how it’s going to go.”

The fight card slated for the Honda Center that night brought a fair amount of firepower in support of the main event.

The undercard included Tito Ortiz dropping a decision to former protege Matt Hamill, who Ortiz believed would be easier to knock out because his deafness had given him “a soft head.” In a welterweight bout, Diego Sanchez and Paulo Thiago earned “Fight of the Night” honors for a three-round battle that ended in a win on the scorecards for Sanchez. The co-main event saw Jake Shields make an unimpressive, but still barely victorious UFC debut against Martin Kampmann after vacating the Strikeforce middleweight title earlier that year.

But it was all preamble to the heavyweight title clash in the top spot.

Velasquez entered first, with a Mexican flag wrapped around his fist. Lesnar strolled out second, brushing aside a cop on his security detail as UFC commentator Joe Rogan chuckled with delight.

“A thousand years ago the only way you saw a guy like this was if he showed up on your shore in a boat,” Rogan told his broadcast partner Mike Goldberg. “And then you ran.”

As the two fighters faced off in the center of the cage, the size difference was impossible to ignore. Lesnar stood a couple inches taller, but his hulking mass seemed to loom over Velasquez. The challenger’s physique did little to indicate his tremendous physical conditioning. In a bodybuilding contest, it was Lesnar in a landslide.

But in a cage fight? That was something different, which quickly became apparent.

Referee Herb Dean gave the signal to fight. Lesnar immediately turned and walked off to one side, as if he’d changed his mind and was looking for the door. Then he turned abruptly and bull rushed Velasquez, driving him backward but failing to take him down in the process.

Lesnar kept digging for the underhooks. He clinched up and threw a series of frenetic knees, at one point leaping at Velasquez, and in the process showing off the spry athleticism that you could almost forget such a big man could possibly possess. When he tried again for the takedown moments later, he got it.

But instead of allowing himself to be smothered by Lesnar’s bulk, Velasquez pushed off on the champion’s hips and scrambled back to his feet. Lesnar worked even harder to haul Velasquez down a second time, but that one was even more short-lived. When he had to stand in the pocket and trade punches with the swift challenger, his long arms seemed to slide past Velasquez’s head just as the counters came thumping back in return.

And then something seemed to shift. Lesnar’s punches slowed. Velasquez slipped under and got a takedown of his own off a single-leg. As Lesnar struggled to get up from his knees, Velasquez chipped away at him with short, quick blows. By the time he made it back to his feet, the champion’s face was smeared with blood from a cut under his left eye. Even worse, now he had his back against the fence, and it was Velasquez on offense.

The first sign of real trouble came when Lesnar went lunging for a desperation takedown and then tumbled all the way across the cage as Velasquez shrugged him off. By the time he regained his balance Velasquez was on him, swarming Lesnar with a combination that convinced him to duck his head just in time to catch a hard knee from Velasquez directly to the face. A clearly wounded Lesnar jogged off to one side and caught a hard right hand behind the ear that put him down.

Then it was panic mode. The first round of the Carwin fight all over again, only this time his pursuer was measured and patient. Velasquez didn’t sprint blindly toward a finish. Instead, he hunted calmly for it, peppering Lesnar with short punches and elbows while he lay on his back, then shrugging off the takedown attempt and stinging him with a sharp right once Lesnar got back to his feet.

Bloodied and wincing in pain as he covered up and rolled from one side to the next, trying to escape the storm, Lesnar couldn’t stop the onslaught. With 48 seconds left in the first round, Dean stepped in to shove Velasquez off. No more. That was plenty.

Velasquez walked to the center of the cage, looking up to the sky with his hands first in the air, then resting almost in disbelief on his own head. He even smiled through his Mexican flag mouthpiece, a rare enough occurrence that it had to mean something special.

Lesnar stood up and wavered from one foot to the other as a cutman worked on the gash below his eye. Blood from the wound decorated his enormous shoulder as he lowered himself onto his stool. In the post-fight interview, Rogan would offer his confident opinion that Lesnar would come back stronger as a result of the loss.

“That’s what a champion does, right?” Lesnar responded.

As he exited the cage, a fellow pro wrestling star was waiting. Mark Calaway, better known as “The Undertaker” of  WWE fame, was halfway into an interview with Ariel Helwani when Lesnar passed him on the way back to the locker room.

“You want to do it?” Calaway asked.

Lesnar just looked at him and kept walking. Whatever “it” was, he seemed uninterested for the moment. As Lesnar would later explain, the timing could have been better.

“Cain put me on a street that I didn’t know the name of, so I was looking for my way home,” Lesnar said.

In a way, maybe he found it. Lesnar would never again hold a UFC title. After being further hampered by diverticulitis he would only return to the UFC a little over a year later, suffering a quick TKO loss to Alistair Overeem at UFC 141. Then it was back to pro wrestling for him, a stint interrupted only by a brief return to the UFC that lasted all of one fight, which was enough for Lesnar to fail two drug tests and get slapped with a suspension after having his win over Mark Hunt at UFC 200 overturned.

While the fight signaled the beginning of the end for Lesnar, it seemed like only the beginning for Velasquez. But when he showed up for his first defense in the very same building the next fall, he was a shadow of his usual self. Hampered by injuries, he was reluctant to pull out of a title fight with Junior Dos Santos that was set to be the only fight on the UFC’s maiden FOX network broadcast.

The fight lasted 64 seconds, ending with Velasquez face-down on the mat, all in front of a network TV audience of nearly nine million people.

He’d get the title back the very next year. He would even take the best two out of three against Dos Santos. But a poor performance at high elevation against Fabricio Werdum in 2015 took the title from Velasquez, and injuries have hampered him ever since.

Victories seemed to come easy when he was healthy. But then? It was like Lesnar himself said at the at press conference just a few days before Velasquez sent him on a downward spiral.

Especially in a sport like MMA, Lesnar said, “if you don’t have your health, you’re not fighting.”

What he couldn’t have known then was just how fleeting good health would be, both for himself and the man waiting on the other side of the podium.

“Today in MMA History” is an MMAjunkie series created in association with MMA History Today, the social media outlet dedicated to reliving “a daily journey through our sport’s history.”

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