The second season finale of "Downton Abbey" aired on PBS this Sunday to impressive ratings (or, at least, impressive by PBS standards). Around 5.4 million viewers tuned into the two-hour broadcast, making it the network's most watched single draw since the first episode of Ken Burns' "National Parks" in 2009.

In recent months, the series has become a cultural juggernaut, raking in accolades—including both the Emmy and Golden Globe for Best Miniseries—and inspiring zealous devotion on both sides of the pond.

So why is a British costume drama, broadcast in the U.S. as a Masterpiece Classic, attracting so much attention?

In short, "Downton Abbey" is a guilty pleasure wrapped in the mantle of a highbrow period drama. A large part of the show's appeal is its plot, which—despite its Edwardian fashions and impressive country manor setting—remains deliciously, unabashedly soapy.

It's also, quite simply, good storytelling. The series' 18 episodes move along at a lively clip; the cinematography, art direction, and costuming are all beautiful.

"Downton Abbey" follows the many inhabitants of the eponymous estate: both the aristocratic Crawley family upstairs and the legion of servants downstairs. The story has, over two seasons, sprawled across nearly a decade, from 1912 to 1920.

The central tension stems from the question of who will inherit Downton—the heir presumptive died on the Titanic—but all the business about entails and dowries is just scaffolding. What's really interesting is the sibling rivalry, class tension, and sexual intrigue (including lovers dying in flagrante) that plays out within Downton's walls.

It is also devastatingly funny, thanks in no small part to Maggie Smith's brilliant portrayal of the Dowager Countess of Grantham, who freely dispenses cutting one-liners and endlessly quotable bons mots.

But the real appeal of the series is rooted in the fact that we, as viewers, have unlimited access to all of Downton and its inhabitants' secrets. Concealment and revelation are constant motifs, undertaken to save relationships and reputations. So too is eavesdropping; hardly a conversation takes place without an uninvited listener one room over. "Downton Abbey" invites us to listen in as well; it cracks the door and pulls up a chair for us.

Of course, the show's tendency toward soapiness occasionally takes a turn for the absurd, particularly in its second season. Both an amnesiac burn victim and a Ouija board make appearances, and during the finale, I half expected the Dowager Countess to wake up and realize that it was all a dream. In Lady Mary and Cousin Matthew, "Downton Abbey" has its own version of Ross and Rachel; their courtship is also prolonged and exhausting. The trials of the upstanding valet, Mr. Bates, are similarly repetitive—at least until he gets put on trial.

The fact that "Downton Abbey" is a costume drama, however, allows pleasure without guilt, even in the face of its occasional overwrought silliness. It takes up contemporary issues—like the First World War, the burgeoning feminist movement, and the shifting boundaries between classes—examines them lightly, and then sets them back down. Lady Sybil flirts with radical politics and the socialist chauffeur, and Downton becomes a convalescent home for wounded soldiers. This engagement with history allows the illusion of respectable intellectualism, even though it functions primarily as a vehicle for more melodrama and plot twists. If "Downton Abbey" has any social commentary to offer, it's well concealed.

"Downton Abbey" is similar to other buzzy shows like "Breaking Bad" and "Mad Men," but only in the sense that everyone seems to be talking about it: fashion magazines, The New York Times, our parents. There are no antiheroes like Walt White or Stringer Bell here; there's no gritty realism, no questions raised about the nature of morality. The viewer doesn't have to do any intellectual heavy lifting to enjoy "Downton Abbey" fully. But that's O.K.: it offers us escapism, beautifully packaged—and that's enough.