The marvelous one has put himself in jail again, this time at the Americana Canyon Hotel and Racquet and Golf Resort. You see his painfully bald head and how it catches the noonday sun, flashing like a heliograph as he walks through the scramble of bodies absorbing cocoa butter at poolside. He finally comes to stand before a table of friends feeding on charbroiled burgers and drinking spirited concoctions of ice and fruit pulp. They offer a patio chair but he seems not to hear.

"Where you headed, champ?"

"No place."

"You want some food?"

"I already ate."

"How 'bout a drink."

"I drank, too."

And you see him very early in the morning, running down roads at the foot of a glorious desert mountain range. And at noon, sitting in a white wicker chair in the hotel fern bar, explaining the grim prison of his mind to a visiting corps of writers, and why he, Marvelous Marvin Hagler, will sacrifice and suffer until the night of April 15, when he defends his world middleweight crown against Thomas Hearns at Caesars Palace in Las Vegas.

"I'm not here to be nice," he says. "This is jail. I'm here to work."

So every afternoon, he returns to the idle ground behind the hotel convention center and spends two excruciating hours in a circus tent, sparring with men who, in style and physical stature, bear an uncanny likeness to Hearns, the World Boxing Council super welterweight champion.

The best of the camp fighters, Bobby Watts of Philadelphia, won a 10-round decision against Hagler in January 1976, one of two losses in Hagler's 12-year professional career. At a workout last week, the champion, who is 60-2-2, bullied Watts around the ring, slapping him into a padded turnbuckle and daring him to strike back.

"Come on," he said through a plastic mouthpiece. "Hit me. I want you to hit me."

Earlier in the day, Hagler had said, "If I was going to fight a Marvelous Marvin Hagler, I wouldn't. I said that under my breath once and my manager heard me. I was looking in the mirror. 'If I was going to fight me,' I said, 'I wouldn't do it.' 'You know, Marv,' he said, 'I heard you. I heard what you said. And it's the truth.' I get stronger, I get meaner. Somebody's trying to take something from me. And I've got to prove I'm the better man."

Hagler, 30, has stressed the importance of maintaining his distance here at camp. Except for Pat and Goody Petronelli, the first his manager and the second his trainer, there is no entourage. Even sparring partners keep a step or two behind the champion, as if determined to remain less an ally than an enemy. Hagler spends much of his free time in his hotel suite, watching TV and writing letters to his family. Sometimes, he said, he "takes little walks in places where people can't find me," all the while regarding the jacuzzis and tennis courts and 18 lush holes of golf at his door as "unmeaningful distractions."

On his first week in town, Hagler rented a fine European touring car but returned it after two days. "It was too soon for all that," he said. "I'm not here to showboat. I'll have my fun later, after I get rid of Tommy."

Last Friday, he and the Petronellis went to Kirk Douglas' house and watched the Larry Holmes-David Bey heavyweight bout televised on Home Box Office. "He seemed like a regular human being," Hagler said of the actor. "None of that Kirk Douglas stuff. He even invited me to sit on his bed and watch the fight."

Hagler had planned to spend a few weeks at his regular training site in Provincetown, Mass., before relocating in Palm Springs, more for superstitious reasons than for the privacy a deserted resort hotel and beachfront afford. The Petronellis feared working out in the unpredictable winter climate at Cape Cod and persuaded Hagler to break tradition.

"People were telling me that because I didn't go to Provincetown things would be bad," Hagler said. "I know superstitions can hold you back and I can't let anything interfere with what I have to do . . . I know this hotel looks real fancy, but not in my head. It's the same with Provincetown. People say, 'Ah, nice, look at the beach, look at the sun.' It ain't pretty, though. Not to me. It's all the same prison."

Hagler holds a mental image of Hearns walking down the beach in Miami, where Hearns has set up camp. Hearns has his shirt off, a troop of mindless sycophants clinging to his new-found celebrity like sucklefish on the belly of a great blue whale. "Tommy's got all those bodyguards around him," Hagler said, completing the picture. "He's throwing his money around, buying Rolls-Royces like a little kid would do . . . I tell you, I don't like Thomas. I don't like his personality. From the opening bell I want to eat (him) alive. You know how Pac-Man does it. Arrunnnch. Arrunnnch. I'm still coming, I'm still coming. Arrunnnch. Arrunnnch. You never know when I'll drop the bomb. All you know is that I'll drop it."

On more than one occasion, Hearns, whose only loss came against Sugar Ray Leonard almost four years ago, has said he would knock out Hagler in three rounds, using his superior height and reach to "outbox" the champion. Hearns, 26, loves to compare his performance against Roberto Duran with Hagler's. It was Hearns' right hand that devastated Duran in the second round; the fight ended when Duran, bleeding profusely from a raggedy cut over the left eye, failed to pull himself off the canvas floor.

In November 1983, Hagler went 15 tough rounds with Duran and nearly was upset. After 13 rounds, Hagler trailed on the officials' scorecards but rallied in the final two rounds to win by unanimous decision.

"Tommy says I'm finished by the third round," Hagler said. "All that means is that he has to show up for the fight. He's still living on the Roberto Duran situation where he got lucky. He caught Duran in the temple and you can't shake that shot. You can't shake that or a belly shot. You can hit somebody in the head and put him on Queer Street for a minute and hit him again in the head and he'll come right back. There's a technique to knocking someone out. It comes usually without you knowing it. If you plan on it, it never comes."

Hagler should know. Of his 60 victories, 50 have came by knockout, and he has not lost in nine years. Even with a remarkable record, he feels his day has been too long in coming and wonders why the strength of his game was so widely questioned after the Duran fight. Preparing for what promoter Bob Arum called "the biggest fight since the first Ali-Frazier," Hagler suddenly finds himself a betting underdog.

"Tommy Hearns is a great fighter," Pat Petronelli said. "And that comes from my heart. We know we're going to have one helluva fight with the guy. You ask a hundred people in the street, and 50 will tell you Hagler (will win), the other 50'll say Tommy Hearns."

Arum agreed. "I'll spend a few days watching Marvin work out and decide there's no way he can lose," he said. "Then I'll go to Miami and watch Tommy Hearns train and figure there's no way he can lose, either. It's that kind of fight."

For Hagler, the bout represents more than a celebrated title defense and a $5.1 million purse. He hopes to parlay the storm of media attention into a movie career and has already met with agents in Los Angeles and New York to discuss the possibilities.

"The public recognizes me as the bald-headed fighter," Hagler said. "Believe it or not, I can wear a hat and shades and an overcoat and they still pick me out. I wonder, 'Is it the way I walk or look? Do I look like a fighter?' I don't think I look banged up yet or anything. At least I hope not."

The Petronellis love telling the story of how they discovered Hagler, or how he discovered them, and sometimes merely shake their heads when asked to describe the odds Hagler overcame to achieve such enormous wealth and stardom.

"When a kid walks in the gym, you don't know what you got," Goody Petronelli said. "Marvin kept coming back day after day and sitting in the same seat and watching everybody fight. One day I said, 'Hey, kid, wanna learn how to fight?' He said, 'Yeah, man.' "

This was in a small gym in Brockton, Mass., home of legendary heavyweight Rocky Marciano, who retired undefeated in 1956 and died in a plane crash 13 years later. Pat Petronelli said he and Marciano had been close friends and "used to work together as kids, putting in sheet rock in buildings, laying down bricks and doing regular construction-type work. We came from the same neighborhood. We used to run together. We'd go to this club and drink a little and dance. We were all just regular working people."

Hagler grew up in a tenement slum in Newark, but his mother moved the family from the ghetto after the 1968 riots turned their home into a bloody war zone. Soon after, they moved to Brockton, where there were relatives, and Marvin dropped out of school in the 11th grade and found work in a tannery.

"I had a little boy," he said. "My mother told me, 'If you quit school, you're going to work and take care of that baby.' In the beginning it was very tough. I had to learn how to talk. I got my education traveling to different places, but not through books. I saw the world. I saw the pyramids. And none of it in books."

Not so long ago, Hagler took his family to the ghetto where he grew up, showed them the mean streets he knew as a boy. During the riots, his mother Mae had made her children sleep under the beds to keep from getting hit by the bullets spraying the windows. Hagler saw the building where they'd lived, the place where his uncle had wrestled him to the floor and shaved his head with a razor, because, he said, "It was inexpensive and it was the style." Hagler, who now has five children, was lost in the memory when his little girl said, "It stinks here, Daddy," and asked to go back home.

Hagler said, "It's easy to get up for this fight. It's the biggest of my career. As far as money goes, this is it. But I still love the boxing game. So it's not the money that's the main focus. I love to box. I love the challenge. I love it when a guy says he can beat me and I know I'm the best. It gears me up.

"People call me all kinds of names. They say I'm slipping, they say I'm an old man. And yet, I'm still the winner. I'm the champ. I'm the only kind of man that I can be."