Unlucky 13, in which Brandon gets a letter from London and has to cancel the outing to Whitwell to ride off on a secret mission. Since he was the ticket to their entry (it belonging to his relative), if he can't go, nobody can. Everyone else goes for a drive, and Willoughby and Marianne head off uninvited and unchaperoned to tour Allenham, the house belonging to Willoughby's aunt. Scandalous.
Here's why Colonel Brandon's behavior is shocking, so you get some idea exactly how shocking it is:
1. Let's remember that Brandon has been firmly established as everything that is right and proper. He is pretty much the perfect gentleman - he isn't pushy or rowdy like Sir John, he's not overly familiar, outspoken or (as we find out later in this chapter) prone to inappropriate unchaperoned/uninvited house visits like Willoughby. Heck, just last chapter the narrator told us that Brandon was "on every occasion mindful of the feelings of others".
2. The arrival of mail during breakfast is a commonplace event in Austen's novels as well as in many other novels written (or set) in the same time period. You may recall that in Northanger Abbey, Catherine Morland got a particularly unhappy letter from her brother during breakfast - she sat through the meal, upset but not wanting to be rude by getting up from the table. She waited to rush off until after General Tilney got up. Not so Colonel Brandon - he scans the letter, becomes visibly upset and rushes out of the room without so much as a "by your leave." I've always figured that part of the reason he rushes out is that he's so upset he can't hold it together, but given what happens next, I rather suspect that there's a double purpose. This marks the first appearance of Colonel Brandon, Man of Action - in the five minutes that he's out of the room, he has pulled himself together, ordered his horse, and undoubtedly instructed his servant to pack up his stuff and make his way to London with Brandon's belongings.
3. Brandon organized the outing in the first place. Though he is a guest at Barton Park, he is the one who has invited everyone to accompany him to visit Whitwell. People have gotten up early and made their way to Barton Park just for this purpose, including the two older Dashwood sisters and two other young ladies. And he has cancelled their outing immediately prior to their departure for Whitwell.
4. While Sir John is sometimes pushy and Willoughby is sometimes inappropriately outspoken, their suggestions here are not completely out of bounds - asking Brandon to hold off on leaving for London for a day, or at least for 6 hours, would not usually have been a big deal. That Brandon is unwilling to defer his departure for any longer than it takes to get his horse saddled shows how desperate he is to be underway, and belies his statements that he's attending a simple matter of business.
Oh, Mrs. Jennings! She is such a busybody, and she really seems unable to help herself. She attempts to directly pry information from Colonel Brandon, which represents an appalling lack of tact and manners on her part. Her daughter attempts to check her behavior, but to no avail.
You will notice, however, that Mrs. Jennings makes what will prove to be an accurate guess as to what the "business" is and who it concerns. She references a "Miss Williams", a young woman who is Colonel Brandon's ward. Once Brandon has departed, she tells Elinor that Miss Williams is Brandon's "natural daughter" - a term meaning that she's illegitimate. (Highly salacious gossip - and whether true or false, it's bad ton to engage in such tongue-wagging, but it is what we've come to expect from her.)
Not content with Willoughby's and Marianne's sketchy account of their hours apart from the group, Mrs. Jennings actually goes to Willoughby's groom to find out where they went. This is both inappropriate and hilarious. I cannot help but admire Mrs. Jennings's tenacity, and I happen to love her (having read the entire book before), so even though I have to point out how shocking her behavior is in encouraging Willoughby's servant to tell her tales out of school (as it were), I am amused by her. Plus we get such good information this way!
For a rest from all this text, here's a bunch of stills from the 2008 production of Sense & Sensibility, which shows Marianne and Willoughy visiting Allenham. The video wasn't findable, alas:
Why Willoughby's and Marianne's conduct is scandalous
1. They mutter at the table about Colonel Brandon being a deliberate killjoy and a self-centered hypochondriac while he can see and possibly hear them, and certainly Elinor and possibly others could hear them. (Brandon is pretty clearly not the self-centered one here, despite his actions. Willoughby and Marianne, on the other hand, deserve that label and worse.)
2. Willoughby and Marianne take off on their carriage ride before everyone else is settled in their carriages. They deliberately avoid the group outing, and it would appear that they were unchaperoned for quite a number of hours, as they were first to leave and last to return. Unmarried couples were supposed to be chaperoned at all times - even openly engaged couples were "supposed" to be chaperoned, although they were often allowed greater leeway (hence we have Elizabeth and Darcy "chaperoning" Jane and Bingley in Pride & Prejudice). Their decision to set off on their own flouts convention and is a decided eyebrow-raiser.
3. Marianne goes to visit Allenham - touring not only the grounds, but also the inside of the house - without having been introduced to its owner, Mrs. Smith, and without an invitation from Mrs. Smith. It appears that she does not meet Mrs. Smith during this visit with Willoughby. Highly improper conduct, and even Marianne (who has not been out in society all that long) ought to know better.
Here's a bit of the conversation in which Marianne claims no wrongdoing:
"I am afraid," replied Elinor, "that the pleasantness of an employment does not always evince its propriety."Yeah, right, Marianne - since you had fun, it couldn't have been improper, because everyone knows that when you're doing something naughty, you can't possibly have fun. Sell stupid somewhere else.
"On the contrary, nothing can be a stronger proof of it, Elinor; for if there had been any real impropriety in what I did, I should have been sensible of it at the time, for we always know when we are acting wrong, and with such a conviction I could have had no pleasure."
When Elinor points out that even if the house were to be Marianne's own house some day (meaning that she was to inherit it from Mrs. Smith, not that she's going to marry Willoughby, although there's a bit of double meaning here as well), Marianne would have been in the wrong to go there without an introduction to and invitation from Mrs. Smith, Marianne finally realises that she overstepped the bounds of propriety - but she is not actually apologetic about it. Instead, her excitement about the house spills out. Marianne's exuberance is bound to get her in trouble. You can just see it coming, even if you don't yet know how things will go wrong.