Jeff Kemp following in father's footsteps as pro quarterback

One of the oldest but most successful plots in show business is the understudy who gets his or her chance when the lead is unable to go on, and, by the time the pit orchestra has put away its instruments, the newcomer has become a star.

This will introduce 25-year-old quarterback Jeff Kemp of the Los Angeles Rams , who has a personal 3-1, won-loss record after taking over for the injured Vince Ferragamo in the team's fourth game of the season. He has currently thrown 81 consecutive passes without an interception and is rated the National Football League's fifth best passer.

Discovering Kemp in the driver's seat is a little like waking up after election day and finding that both Ronald Reagan and Walter Mondale have lost to a write-in candidate. But the Rams' three top wide receivers aren't complaining. Since Kemp took over, they are all averaging 22 yards per catch.

Kemp is so ruggedly wholesome that he looks as though he just fell off a cornflakes box or could apply for a Malibu lifeguard's job. After throwing just 31 passes in his first three years with Los Angeles, the Rams have entrusted him with their immediate future. From here the whole thing has the look of a permanent relationship.

Of course when Jeff finished his gridiron career at Dartmouth in the winter of 1980, National Football League scouts figured him for a three-piece business suit and attache case. Jeff's experience had been stricly Ivy League, where they don't exactly keep their linebackers in cages between games, and where they haven't hung a losing coach in effigy in years.

The poet in the press box who listed Kemp at 6 ft. 1 in. in Dartmouth's football guide may have used a rubber measuring tape. During his varsity seasons at Hanover, N.H., he threw the grand total of 13 touchdown passes and graduated with a degree in economics. The football staff hadn't even taught him to grunt, and worse yet he was the son of US Congressman Jack Kemp.

However, if you remember Jeff's father at all, you also remember a quarterback who gave the old American Football League a lot of credibility in its early years. In fact, Jack led the Los Angeles (San Diego) Chargers to 10-4 and 12-2 seasons in 1960-61, although they did lose twice to Houston in the AFL's championship game.

Through a clerical error in the Chargers' front office that left a temporarily injured Kemp unprotected, Jack was claimed on waivers by the Buffalo Bills, a bargain akin to the purchase of Manhattan by Dutch settlers.

During his seven years at Buffalo, Jack twice led the Bills to championships while throwing for more than 15,000 yards and 77 touchdowns. Even though most people probably think of former Minnesota QB Fran Tarkenton as pro football's best scrambler, Kemp filed a few patents of his own on avoiding onrushing linemen.

When the Rams finally got around to signing Jeff Kemp to a contract in 1981, it was as a free agent. They weren't even sure he was a quarterback; something about his throwing arm maybe not being strong enough for the NFL. There was also some talk about him being looked at as a defensive back because of his quickness.

Anyway, while they were trying to decide what to do with Jeff, the Rams kept sending out people named Pat Haden, Bert Jones, Vince Ferragamo, Bob Lee, Jeff Rutledge, and Dan Pastorini to play quarterback for them. Haden had some good games, but took early retirement after the 1981 season.

When Ferragamo decided he didn't like the salary figures on a new contract, he took his talents to the Canadian Football League where he never did adjust to the oversize field and fewer downs. While Pastorini, who was brought in to replace Vince, once had the golden touch as an NFL passer, by the time the Rams got him he was yesterday's mashed potatoes.

Meanwhile Ferragamo, free of his Canadian obligations by 1981, but still LA property, asked his old team to take him back. In what was one of the fastest cases of joint forgiveness on record, Vince got his job back. And while he fought injuries in 1982, last year, under new Coach John Robinson, Ferragamo took the 9-7 Rams to the second round of the playoffs.

If you are wondering what Kemp was doing during this period, he was listening and learning and wearing a set of headphones that gave him a direct connection to the team's coaches in the press box. While it wasn't exactly on-the-job training, he did learn to read the keys for just about every pro defense in the book.

Asked about Kemp after he had thrown three touchdown passes last week against the New Orleans Saints, Robinson replied: ''So far, Jeff has been able to make the big plays for us, and when you consider how little playing time he had prior to this season, he's done a remarkable job. The fact that this kid can scramble if he has to also gives our offense a lot more punch, and for a couple of weeks anyway most teams didn't realize that he could throw deep. However, by now we've lost that element of surprise.''

As for Kemp, he says pro football has been a week-to-week learning experience for him since taking over for Ferragamo, who may be soon be ready to play again.

''Maybe the biggest thing I've discovered since I became a starter is that when a Ram receiver gets in a position where he only has to deal with one defender, the quarterback better find him and get him the ball,'' Jeff explained. ''That's the kind of thing you talk about all week in practice, but that doesn't become part of your instincts until after you've played a few games.''