"You’ve got to see this tree!” calls out Sforni’s husband, Charles Firmin-Didot. Sunlight glints off Pennypack Creek, which meanders through the 2.6-acre property in suburban Philadelphia. The white flowers of the silver lace are just beginning to bloom, and already their sweet fragrance perfumes the air. As we arrive at the must-see tree, a magnificent, 70-foot-tall tulip poplar, Sforni places her hand on it reverently.

This walk in the woods wasn’t what I expected from my visit to the couple’s house, a 1967 masterpiece by the great Modernist architect Louis Kahn. But considering that much of the best Modern architecture—and Kahn’s work, in particular—is about connecting with the outdoors, our quick hike feels like the most appropriate introduction imaginable. The towering tulip poplar was just a sapling when Dr. Norman Fisher and his wife, Doris, contacted Louis Kahn in 1960. Residents of Hatboro, Pennsylvania, the couple had purchased a lot just half a mile from their existing, late-Victorian residence. They wanted a Modern house, and another architect had mentioned Kahn. So they found him in the phone book, called him up, and asked him to design them a relatively modest Modern home.

By this point in the Philadelphia-based architect’s storied career, he was designing high-profile, far-flung buildings, including the revered Salk Institute for Biological Studies in La Jolla, California. He still loved to create houses for the right clients, though, and he must have sensed kindred souls in the music-loving Fishers. He spent four years designing their house, going through four different schemes before settling on one that would both accommodate his clients’ budget and satisfy his exacting eye. The final plan was beautifully simple: two connecting cubes—one for sleeping and the other for living—set at a 45-degree angle from one another.

Clad in cypress and set on a foundation of local Montgomeryville stone, the painstakingly designed, 1,800-square-foot house took three years to build—an unusually long time for a structure of this size. But the care and time taken by builder E. Arol Fesmire paid off. Nearly half a century after completion, every handcrafted kitchen drawer slides out of its oak casing like silk. As for Kahn’s deliberate pace with the design process, the Fishers’ daughter Nina doesn’t recall her parents complaining. “I think they recognized they were working with someone who didn’t operate the way other people did,” she says.

For almost 40 years, the Fishers happily inhabited the results of Kahn’s (and Fesmire’s) labors. Their friends didn’t always like the look of the house at first—it sits,discreetly, on a street of more conventional Capes and bungalows—but according to University of PennsylvaniaArchitectural Archives curator William Whitaker, it grew on them. “Doris said she could see a change in people over time,” he says. “The house would grip you. It was about connecting at a deeper level, which is what Kahn’s architecture is about.Kahn designed more than 30 houses, just nine of which were built. (All nine are still standing.) The Fishers never underestimated their responsibility as the owners of one. They kept the building in immaculate shape while raising Nina and her older sister, Claudia, as well as a series of dogs, mostly Irish terriers. “Particularly after he retired, my dad poured his time into taking care of that house,” Nina recalls. “He delighted in showing people around.”

Norman died in 2007. The following year Doris moved to Annapolis, Maryland, where she still lives, to be closer to Nina. In 2011, the family decided it was time to let go of the house. “In a way, it was a relief to be able to turn it over to a responsible party,” Nina says.

The National Trust, which had spent more than a decade assessing the house, decided that the best way to take care of it in the long term would be to sell it to a private owner. The buyer would be bound by the easement’s remarkably comprehensive terms.

“It provides a level of curatorial protection that is unusual,” says Shantia Anderheggen, director of easements at the National Trust. No changes are permitted to the building’s exterior, which is typical— but few are allowed throughout the interiors, either, which is out of the ordinary.

After working closely with the Fisher family to further refine the easement, the National Trust placed the house on the market early in the summer of 2012. Interested buyers were asked to submit sealed bid offers by the end of August. The day before the deadline, Anderheggen received a scratchy-sounding overseas phone call from a Charles Firmin-Didot. “He sounded like he was calling from the Titanic,” Anderheggen says. “He knew who Louis Kahn was.” Better yet, he didn’t want to change the house at all. Firmin-Didot had also started restoring his family’s 16th-century farmhouse in Normandy, France, and had owned and maintained other old houses. He was exactly the type of buyer the National Trust and the Fishers had been waiting for.

A clip from the Oscar-nominated 2003 documentary My Architect, made by Louis Kahn’s son Nathaniel, shows Kahn talking about serendipity. “How accidental our existences are, really, and how full of influence by circumstance,” he says.

The series of events that led Firmin-Didot and Sforni to the house underlines Kahn’s point. Sforni is a fine-arts photographer with a studio in New York, and Firmin-Didot worked in London as a scout for entrepreneurial talent. The couple happened to be visiting a friend in Philadelphia while Fisher-Kahn was on the market. Their host took them to a weekend open house there, just out of curiosity and general interest. “Our friend said it was an incredible opportunity to see a Kahn,” Firmin-Didot says. “Then the house was so beautiful it stayed in my mind.” He loved it, and so did Sforni. But they didn’t seriously consider buying it. “I didn’t have the money available to buy this house at the time, and I had work in Europe, so it didn’t make sense,” he says.

But fate intervened. Late that summer, Firmin-Didot concluded his employment in Europe, accepting a package that happened to match the house’s asking price almost exactly. In another cosmic coincidence, the couple realized that a specialized photo lab Sforni had worked with for years, the only one she’d ever found that printed her images just the way she wanted, was located about 20 minutes away from Hatboro. (Alas, it has gone out of business.) “I kind of respect when there are signs like this,” Firmin-Didot says. “The house chose us.” He quickly made the call to Anderheggen and sent the paperwork detailing his and Sforni’s desire to preserve the building in accordance with the easement. The Fisher-Kahn House had found its next owners.

Today, after my woodland walk with Firmin-Didot and Sforni, we sit down to a delicious lunch of roasted chicken, hard-boiled eggs, potatoes à la dauphinoise, and a simple green salad. Whitaker joins us on his way back from a visit to another Kahn-designed residence, the Korman House in nearby Whitemarsh Township, where he’s advising the owners on some preservation work.

Firmin-Didot and Sforni have prepared our meal in their vintage 1967 kitchen. “It’s a perfect-size kitchen for someone who likes to cook,” Firmin-Didot says. The stainless steel counters were designed an inch or two lower than usual to accommodate Doris Fisher’s height of 5 feet, 2 inches. Kahn specified handmade, dark red floor tiles, which remind Sforni of her native Italy.

We eat at a 1964 George Nakashima–designed walnut Trestle Table, chosen for its eastern Pennsylvania provenance and its kinship with the house’s architecture. After they bought it, Firmin-Didot and Sforni learned that the Fishers had had the same one. The smooth wood table sets off the textured walls, which are covered in a mixture of lime and plaster painstakingly concocted by Kahn.

According to the easement, the lime plaster can never be painted or otherwise altered—to do so would remove a fundamental piece of the Fisher-Kahn House’s character. The hand-worked, slightly rough surface gives the house an ancient feeling, like an old French farmhouse or even a Greek temple. It catches and transforms natural light in an almost uncanny fashion. Before lunch, the walls looked off-white in some places, gray or light brown in others. Now they’ve turned a gentle yellow and green, reflecting the leaves outside. In some places, they’re washed with pink from the reddish hue of the cypress window frames.

The lime plaster walls capture not only still light, but movement and shadows, as well. Firmin-Didot shows me a video he took of the early morning light in the master bedroom: On the ceiling, an eerily beautiful river of moving shadows reflects the flow of Pennypack Creek. And in the dining space, Firmin-Didot and Sforni often move the Nakashima table aside and project movies onto the wall.

Sforni, a philosophical sort like her husband, tells me she sometimes enjoys just sitting inside and watching the day change around her. “It’s such a pleasure to be here all day,” she says, noting that the house helps her to recharge and gain perspective. “You don’t want to leave.”

I can identify with that.

After lunch we gather around the massive fireplace, with its half-round Montgomeryville stone chimney. Kahn had the masons rake out the mortar joints to give it a dry-laid appearance, like a Scottish cairn. Sitting at a built-in window bench, Firmin-Didot and Sforni tell the story of their first visit to the house as owners.

Hurricane Sandy hit New York, where the couple was staying at the time, on October 29, 2012, essentially shutting down much of the city. As soon as the tunnels out of Manhattan opened up, they drove down the New Jersey Turnpike to Hatboro, wondering uneasily how much damage their newly purchased house had sustained. When they arrived they found that trees had fallen all over the neighborhood, but despite its wooded setting, the Fisher-Kahn House remained untouched. Power lines were down in the area, but their electricity worked perfectly. They lit a fire in the fireplace and turned the thermostat to 69 degrees, and the house was warm in 15 minutes. Firmin-Didot and Sforni took their incredible luck as another sign that destiny had aligned them with Fisher-Kahn. “The house has treated us very well,” Firmin-Didot says.

And vice versa. The only major item the couple had to fix upon moving in was a leaky pipe. Since then, much like the Fishers, they revel in their role as maintainers and stewards of the house. “For us it’s so natural to respect materials, so it doesn’t feel like a weight,” Firmin-Didot tells me. “You have responsibility, for sure, but it feels natural.” Later, he falls deep into conversation with Whitaker about the best approach to preserving the cypress siding while also deferring to Kahn’s original vision.

Finding furnishings and artwork that suit the house has been a pet project of Firmin-Didot’s. Simple wood items not necessarily from the Midcentury Modern period—an Amish daybed, a coffee table bought on a roadside in Sicily, an African stool—mix with pieces by Kahn’s contemporaries such as Nakashima. A clothand-papier-mâché sculpture of a faceless man by artist Mark Jenkins resides in an upstairs closet, surprising Whitaker and me as Firmin-Didot opens the closet door. “His name is Bryan,” Firmin-Didot says mischievously. The juxtapositions seem fitting given Kahn’s eclectic influences, which included vernacular and classical traditions.

A glass sculpture of an old-fashioned loudspeaker by artist Philippe Parreno occupies a high shelf above the main living space. “It is the voice of Kahn and Fisher,” Firmin-Didot says, joking.

But it’s true, in a sense. The house somehow has its own personality, shaped by the man who designed it and the family who lived there for so long. Playful, empathetic, calm—these are adjectives more commonly applied to people than buildings, but all of them have come up in our free-flowing conversations about the house today. Whether by accident, destiny, or architectural karma, the voices of Firmin-Didot and Sforni are now part of the Fisher-Kahn House, too.

ONLINE EXCLUSIVE: Meghan Drueding delves into what it's like to live in a Louis Kahn-designed house with quotes from people who have called it home (plus more photos).