Ray Allen spends his nights unlocking life's little mysteries. He is summoned at all hours, by callers frantic and embarrassed, apologetic and impatient. Their keys, they explain, are dangling in the ignition of the Mercedes-Benz. Dinner is burning on the stove. They only left for a second, to get the laundry. Now the door won't budge.

After four years at Rollins Lock and Safe Service Inc. in Arlington, Allen picks his way through such emergencies with practiced calm.

He's coaxed open the locks on a boat, an airplane and a tractor. He once picked a car ignition while crouching under the steering wheel, a flashlight pinched under one arm.

In a town of security-conscious professionals, locksmiths like Allen hold a distinct mechanical advantage. They know why your doorknob works and what makes your file cabinet click.

They also know that power, in some cases, may depend on who's holding the keys.

"Sometimes people are so happy to see you -- it's like you're the greatest thing since sliced bread," says Allen. "The simplest things amaze people." Pick a lock, he says, and "you mesmerize the paper pushers. They say, 'Gol-ly!'

"You get the audience every now and then when you're trying to open a car -- somebody saying, 'Aw, my brother-in-law got in there with a coat hanger.' "

Once Allen showed up in a parking lot with his slim-jim -- a narrow, springy strip of metal good for poking into locked cars -- and the car's owner asked in astonishment, "You mean you brought your own coat hanger?"

"Opening a Mercedes-Benz," says Allen, "is not fun. Volvos are not fun. They're tight -- it's hard to get down between the window and the weatherstripping . . . . I like opening cars best at night because you can pull the weatherstripping out and shine a flashlight in there and see everything!"

As long as people have things to lock up, Allen knows he'll never be out of a job. Inevitably, forgetfulness and circumstance will foil the most conscientious lock owner. A brisk wind will sweep the door shut. The keys -- of course -- will be on the hall table or in the pocket of the pants worn yesterday.

Locksmithing "keeps you thinking," says Allen. "You've got to use your head or you end up buying doors for people . . . . It's being a paid burglar, and I don't have to look over my shoulder when I leave a job."

Allen is tall, with a deep brown push broom of a moustache and a frame thin and wiry as a slim-jim, agile enough to scale a third-floor terrace now and then when his lock-picking efforts fail.

"I've laddered up a few. I've climbed up other people's balconies knowing someone's going to look out the patio window and say, 'What are you doing on my balcony?'

"I've never had the cops called on me."

Allen arrives with a can of oil, a hammer, screwdrivers and a worn leather pouch stuffed with little tools, some of which resemble dentist's instruments. Others look like metal inkblots on the ends of sticks. Allen has carried the pouch every day for four years. He says he feels naked without it.

Customers would need Allen's services less frequently if they heeded his advice -- turn a key gently, don't wrench it, and oil all locks twice a year.

"People think they have to wrestle a key," he says. "They come in with their keys all twisted."

He knows that most people don't think much about locks.

"You ask the person which way does it turn to unlock, and if they hesitate and start moving their arm like they're opening the door, then say it turns to the right, I pick it to the left."

Allen works with Mac Cardwell, whom he calls "the inside brains" of Rollins. Cardwell once got a locked Cadillac open in two seconds, a 1978 Thunderbird in three. He picked a pair of government-issue handcuffs. He opened a door in Old Town with his Maryland driver's license.

Cardwell loves locks. He prowls auctions and flea markets looking for old ones, unusual ones, and displays part of his collection on a tattered, purple velvet sheet. He'd like a better showcase for them: "One of those display cases on legs with the Queen Anne feet."

Charles Buck owns the shop. The other man who works there is Dennis Key.

Allen wears a beeper and handles the emergencies.

Sometimes, if he's driving or taking a walk and sees a driver in distress, poking around in vain with a coat hanger, he'll help the person and not charge a dime.

Other times, the locksmiths say, people will call in a panic, and it turns out their key is bent or their lock is rusty.

"There will be an older couple, an older home," says Cardwell. "They say, 'The keys don't work.' You find out the key's a little bent. You squirt a little oil in there. So you don't charge them. They love that. They eat that up."