Close-reading a Downton Abbey B-Plot

Mary and Sybil’s interactions in this week’s Downton Abbey (season 3 episode 2) b-plot were a particularly well-done bit of writing, very complex and real and satisfying, partly because the place where it lands us is for now relatively without consequence in a show that often limits its gratuities to shit the Dowager Countess says.
In Act I, Mary comes upon Sybil and Branson the chaffeur and realizes their attraction.

It’s a natural encounter, in the course of daily household life, and in addition to starting up the episode’s b-plot it prepares us for Mary’s tart line when she confronts Sybil while they’re dressing for dinner:
“That is why one talks to chauffeurs, isn’t it? To plan journeys by road.”
Mary just can’t help herself—she’s smart enough to be bitchy, and so she is, even when she has Sybil’s best interests at heart (albeit presumptuously). Sybil rejoins with principle, citing both her own independence and the wartime blurring of class boundaries—the high ground, with its attendant potential for naivete and priggishness and disregard for consequences, as Mary reminds her.
But in Act II, when the Dowager Countess broadly hints at suspicions regarding Sybil, Mary keeps her sister’s confidence. From the Countess’s phrasing, however (“Mary and I were talking about you the other day,” referring to her questioning of Mary), Sybil infers a conspiracy of her elders, making a confrontation with Mary inevitable.
In Act III, when they hash it out, it’s in the halls of the Abbey, as Sybil goes about her duties as a nurse. The staging adds a wonderful fluidity, the casual interest of the passing rooms, and the suspense of wondering who will overhear what (someone always overhears something). It gives us a chance to hear the doubts that Sybil hasn’t told Branson, and to see a practical side of her. But this time her talk is all personal; no high principles but simply, “I’m not even sure sure I like him.” Mary’s “What did you think, you’d marry the chauffeur and we’d all come to tea?” lands quite differently after that. She’s still pithy, but she’s correct, and she comes across as having considered things, while Sybil seems caught up in and flustered by a girlish dalliance. Mary gives her a frustrated ultimatum: she’ll keep Sybil’s secret as long as Sybil promises “not to do anything stupid.” Sybil agrees, irked, and her needing to be about her duties (implied) naturally close the scene.
At the end of the act (that the plot is resolved before Act IV reflects its relative unimportance, for now) Sybil tells Branson that Mary knows—that she told Mary, not that Mary figured it out or suspected.

Cynically, challenging her to deny it, he says:
“So that’s me finished, then; without a reference”
(we’ve had the too-tidily recent example of Ethel the maid for comparison). Sybil answers:
“No; she’s not like that. You don’t know her. She wouldn’t give us away.”
What a wonderfully complex line. Sybil asserts her independence from his cynicism and affinity with the sister she’s been more or less continuously sparring with and rebelling against. It’s the kind of place you want to get to as a storyteller, a character saying more than he or she realizes in simply describing another character, but with the history behind it to make the statement knot you up inside hearing it, because you know what she’s choosing to tell (or not). The affirmation of their safety frees Branson to take pleasure in her other sentence; it’s the first time she’s said “us.” Even that turn is not allowed to be unturned, and the exchange spirals into the kind of argument—a real couple’s argument—that they need to have. They both pull back into who they are with others, all of which they’ll need to negotiate if their relationship is to blossom outside the particular circumstances in which it began.