Downstairs, Mrs. Patmore is lightly shaming Daisy to Alfred by telling him that she's been such a big help and yelling to her that it's time for "them" to check the pudding; Daisy's "What is this new game" expression suggests that Mrs. Patmore is not going to have any trouble winning this conflict, not that anyone had any doubts.
Upstairs, Lady Cora is sure that Mary's outburst was caused by mere nerves. Maybe she's just trying to calm everyone, but you'd think the fact that they dragged a corpse across the length of the house together would have taught Lady Cora that her daughter's made of slightly sterner stuff. Martha apparently shares my opinion, as she asks of Edith -- who apparently relayed what she witnessed -- what the fight was about. Edith tells them the extent of what she heard, which is Mary's accusation about Matthew not being on their side. While the Dowager Countess thinks that might be serious, Lord Grantham finds it absurd and declares his intention to go see Matthew. Then Branson, emboldened by his formal Dowager Countess-approved induction into the family, pipes up that as his best man, he should be the one to do it. Lord Grantham can barely strangle out a "What?" in response, but Branson explains that while he's not exactly comparing his and Matthew's situations, he knows what it's like to marry into the family and Matthew is an outsider, albeit a different kind. In the context of what Edith overheard, it's a perfectly logical position and Martha puts paid to any further discussion: "He's the one who'll lose his job if the wedding's canceled." That's fair, as long as Branson doesn't remember that sabotaging his own effort will get him out of wearing a morning coat.
Downstairs, Daisy is under the illusion that she's not playing right into Mrs. Patmore's hands by confronting her about "not responding to my protests." Mrs. Patmore merely laughs at her relatively exalted language before "asking" if she's been talking to Thomas and Daisy's face lets us know that she'd never be suited even for gin rummy, let alone poker. In the face of a cross, red-faced woman who's figured out her game, Daisy lasts for about two seconds before agreeing to dry the dishes and Mrs. Patmore tosses it at her with a satisfied expression. All in a day's work, Daisy... not that you'd know.
Mary -- still distraught and having changed for bed -- has told Anna the whole story, and while Anna thinks Matthew has to be true to himself, Mary argues that that's just the point: he puts himself about the family unit. I'm pretty sure that Lord Grantham would agree with Matthew's position if he knew it, but Mary probably would think that's irrelevant; she'd need a woman in the family to disagree for her to have any chance of seeing it Matthew's way. Anna, however, tells Mary that Matthew's a good man and they're not like buses, i.e. "There won't be another one along in ten minutes' time." I might get rid of my car if I lived in a place in which that was a serviceable metaphor; regardless, Anna's words look like they are having an impact on Mary.