Marimón works for Eternautas Tours, an agency that employs historians as guides. With the huge growth of this city, I need someone who will illuminate even places I know. Such as Plaza de Mayo.

Since the time of Juan and Evita Perón, Argentine leaders have convoked supporters to mass demonstrations at Plaza de Mayo. The square's lawns, flower gardens, fountains, and statuary, Marimón tells me, belie its reputation as the "plaza de protestas."

"Everyone has heard of the Mothers of Plaza de Mayo, who started gathering here in 1977 to protest the 'disappearing'—the secret torture and execution—of their sons and daughters by Argentina's military junta. But Plaza de Mayo's notoriety goes back to 1945, when supporters of imprisoned populist caudillo Juan Perón demanded his freedom and changed the nation's history."

On this Monday morning, the only presence is a group of war veterans. Marimón steers me down the Parisian-style Avenida de Mayo to Café Tortoni. The oldest café in Argentina, it is a genteel, wood-paneled landmark that opened with fanfare in 1858. "The Tortoni is emblematic of the elegant cafés that were central to the cultural life of Buenos Aires in the 1920s and 1930s."

That legacy also marks the nearby quarter of San Telmo, birthplace of the tango, even as the area modernizes itself. Cobblestone streets, shops brimming with antiques, and a lively Sunday flea market at Plaza Dorrego make this barrio a must for every visitor.

Marimón dismisses the street of colorfully painted metal-clad houses known as the Caminito—a popular tourist enclave in the working-class barrio of La Boca—for its bad taste. "It mixes every Argentine idiom, from Diego Maradona to Evita to gauchos. It doesn't give a true impression of La Boca, which in the 1800s was the main port of our city." He does approve of Fundación Proa, a waterfront restoration he pronounces "a landmark of the arts scene that appeals to tourists but was not put here for tourists."

In fact, the arts in BA are flourishing. Among the recycled brick buildings of Puerto Madero, "another reconversion of a waterfront area where money is very conspicuous," says Marimón as we pass by, I spot Spanish architect Santiago Calatrava's extraordinary Puente de la Mujer, a pedestrian suspension bridge that hovers like an Art Moderne harp over the yacht basin. Here, also, one of Argentina's wealthiest women founded the glistening new, glass-metal-and-stone Colección de Arte Amalia Lacroze de Fortabat, a museum that focuses on Argentine art.

It's near the lushly landscaped Plaza General San Martín, where independence hero José de San Martín trained troops, that Marimón stops me in front of Palacio Paz. "This was the largest private home ever commissioned in Argentina," he tells me. "The really wealthy gentry tended toward Francophile palaces." Today home to an exclusive officers' club and open for tours, the Versailles-feeling palacio boasts three levels. "One was for social visits; one was for bedrooms [35 in all] and family life; and one was for servants' quarters and the kitchens." Its chandeliered ballroom reminds me that porteños, more than other South Americans, have used Europe as their lodestar.

The Francophile tradition really flourished in Recoleta. "This is the site of one of the world's most visited cemeteries," says Marimón, "the Cementerio de la Recoleta, noted for the crypt holding the remains of Evita Perón." As we roam the alleys and diagonals of this opulent necropolis, it comes across as aggressively secular. "None of the early tombs," Marimón observes, "have crosses or other religious symbols. They're adorned with obelisks and caps, which are republican emblems." In Buenos Aires, even death gives voice to politics.