With a day of stubble on his ruddy face and a paunch above his belt, Michael McKeever looks more like a Law & Order everycop than Tom Selleck. His is the perfect guise for a private investigator: instantly forgettable. We are sitting in McKeever's silver S.U.V. in front of an enormous three-story colonial in central Long Island. The street is wide and tree-lined; kids zigzag over the double-yellow lines on their bikes. Across from us, two ponytailed mothers giggle in a driveway.

We are here to see a woman who, with any luck, won't see us. She is the driver of a white BMW that's parked 150 yards ahead on the opposite side of the street. This morning, the woman's husband called the man sitting next to me and explained his wife's plans to teach an 8:30 p.m. fitness class. He thinks she's lying. They've been married for fourteen years. The husband says he's still desperately in love with her. They have two children, ten and twelve.

Of all the work McKeever takes on, spousal spying is the most unsavory. It's also his bread and butter. More than half of the roughly two thousand cases he has pursued in his thirty years as a P.I. have been at the behest of a suspicious spouse. Most fit tonight's profile: a husband or wife aged thirty-five to fifty-five, married five to twenty years, kids, a mortgage and marital malaise.

Roughly eighty percent of McKeever's spousal clients are women. "Often they already know," he mumbles gruffly, "but they're looking to answer that five percent uncertainty. They want confirmation." He considers his female clients "astute." Nine times out of ten, he says, their suspicions pan out. Usually, they've noticed a specific behavioral change in their husbands: He's suddenly going to the gym. He's started wearing cologne. Some female clients already know that their husband is cheating, and even with whom. They give McKeever her name and phone number. I just need to know what she looks like. Is that strange?

Men are a different story. "They're wrong maybe seventy-five or eighty percent of the time," says McKeever. Their suspicions are typically based on flimsy evidence: I don't know what she's doing while I'm at work. I've got a bad feeling. McKeever spends endless weeks following wives from their homes to the gym to Starbucks and back home again.

Tonight, we've been hired by a husband to tail his wife, which means that, statistically speaking, we'll most likely follow the white BMW to a Curves or a Barnes & Noble, where nothing unseemly will occur. But the number-one rule of private investigation, McKeever tells me, is "never assume." He relates the story of a client who thought his wife was having an affair because her post-work arrival time had shifted from 5:45 to six. McKeever thought the man delusional, but he followed the wife anyway. Sure enough, she jumped into a male coworker's car at five p.m. and was home an hour later. The husband was right.

Now it's 7:09. McKeever palms his digital camcorder, as he will do every few minutes for the rest of the night, and records five seconds of time-stamped footage of the BMW. He will later email the digital file to the client as proof of both his work and the wife's activities. Then we sit. He pulls out binoculars. I pull out my Jackie O. sunglasses. Incidentally, McKeever tailed Jackie decades earlier, when a tabloid hired him to confirm her engagement to Aristotle Onassis.

A couple walking a dog gawks at the binoculars. Had they asked, McKeever would either come clean and say that he's an investigator or claim that he's "waiting to give an estimate." They decide what sort of estimate for themselves. Bystanders make surveillance in the suburbs difficult, but not as tricky as in rural areas, where every passing driver stops to talk ("Y'all good in there? Can I getcha a tow?"). Sometimes McKeever sticks a hard hat on the dashboard so people think he's waiting for his construction crew. Today, I'm the hard hat. A man with binoculars is a perv. A man and a woman with binoculars are birdwatching.

It's surprisingly exhausting to stare at a car 150 yards away for more than an hour. I'm itching to take a stroll, nap, or have dinner. The dashboard clock ticks past eight. At 8:09, a woman in a tiny white dress appears. She hops into the BMW and tears out of the development. "He told me she drives like a maniac," McKeever mutters as we pull out — and nearly lose her on the first turn. Though McKeever's Honda Pilot is nebbish, he's clearly juiced up the engine. He grabs a video shot of her passing a street sign. The phone rings: It's the husband, at home with the kids. She's leaving, he informs McKeever over the phone. "We got her, thanks."

We roar fifty yards behind the BMW as it makes a high-speed turn and loops back into the development. She puts on a turn signal, changes her mind, then flies onto a side street. We fly in behind her. "Oh, shit." McKeever slams on the brakes and pulls behind a parked car. She's making a U-turn, and stops in front of a blue ranch house, facing us.

She's moving around in the car. McKeever stares at her, relaxed. I'm worried she'll notice us, just twenty feet ahead, but McKeever isn't. Clients always tell him, "My wife's really sharp, she notices everything." They don't. If McKeever is spotted, it's usually because the client tipped off the spouse. Some fighting spouses have a habit of shrieking, "I'm sick of this crap, I'm gonna hire someone!" and a week later, the spouse spots McKeever in the rearview mirror.

McKeever drums his fingers on the steering wheel and tells me about working for a wealthy Frenchwoman who hinted daily to her husband that she was having him tailed. "The guy would come out of his office like James Bond, looking for me," he laughs. "One time I'm sitting in front of his girlfriend's house, and I see something behind a tree. It's the guy. There's a truck going by, and the guy is like Steve McQueen, crossing the street behind the truck. I got it on video."

The BMW lurches toward us. It's 8:15. McKeever instructs me to look at him and pretend to talk. I talk. Our woman is now wearing pink, having changed in the car. She guns it onto the main road, and we execute a white-knuckled U-turn and follow her out, just in time to glimpse her making a right at the intersection under a changing light.

The BMW merges onto a highway and spends the next twenty minutes tailgating people. This woman clearly fits a risk-taker profile. Three towns later, she veers off the highway and zags through a quaint main street, sliding into a parking lot behind a supermarket. We pause on the road. She's sitting in the car. We pull in diagonally from her, facing away. The lot is filled with acne-covered teens blasting music from Porsches. It's 8:30 p.m.

I crane my neck to see what she's doing. Is she on her cellphone? Redoing her makeup? Snorting a line? Crying? McKeever keeps one eye on his rearview mirror. He pulls out the photograph from the husband. It's a poorly focused headshot of a copper-skinned woman with disheveled reddish hair, bright pink lipstick and a suit jacket pulled over a low-cut blouse. McKeever studies client photos. "Sometimes you look at the photo and think, 'I don't think so.' And sometimes you think, 'Yeah, probably.' You can't always tell, though. As my mother says, there's a lid for every pot." I ask him why his clients seem to lid multiple pots. "Is it a self-esteem thing?" he asks rhetorically. "I really don't know. It's impossible to figure people out."

Tomorrow, from his office, he can find out who the wife has called lately, and her driving record, using services like IQData.com, Accurint.com and LocatePlus.com, along with well-placed contacts in various agencies and companies. But McKeever will never speak to her. He will learn who, what, when and where. Never why. "I'm a businessman with the job of providing information," he says. "If someone hires me to follow their husband, I don't say 'Why?' I say, 'Okay, where does he work? I need a photograph.' I don't pry."

What McKeever does know a lot about is how people gather information about their partners. "A lot of times, there's this process people go through where first they believe someone blindly. Then they start to wonder. Then they start to scrutinize. And then they really pay attention. Maybe they hire a private investigator. They make a decision."

This process is essential. McKeever was recently hired by a father who had McKeever follow his daughter's husband. The daughter had not yet begun to wonder, and disbelieved McKeever's photographic evidence of her husband's double life. Partners handle information on their own schedule. Most are ready to address the situation by the time they call McKeever. "You don't hire a private investigator to get information that you're not ready to absorb," he says.

Tonight's husband became ready to absorb this morning. McKeever thinks the wife is a serial philanderer, based on his time-proven theory that people who marry partners whose primary virtue is sexiness are more likely to fool around. "If you marry someone that's not so attractive, obviously you see a lot of things other than the superficial," he says. "If the relationship is like an onion — there are so many layers of intimacy, plus the bird watching and intellectual conversation — then you don't need to worry. If you marry Charo, you need to worry." Tonight we are following Charo.