Lady Mary is struggling with the idea of honesty in regards to her feelings for Matthew. Now that he has a fiancee, she feels that the only thing left for her to do is settle for second best, in the form of newspaper man Sir Richard Carlisle, who has proposed marriage to her, not in the name of love, but in the name of a good business/social partnership. Turning to the staff for help on the matter, Carson—who's as invested in the Mary/Matthew love story as the rest of us ladies watching at home—tells her that she must be honest about her feelings, implying that she should confess her love to Matthew, Lavinia be damned. Anna—who's all high-and-mighty about her own much less exciting/much more annoying love story with Bates—essentially tells Mary that she might regret it if she lives a lie with Carlisle.
So off Mary went to see Matthew and tell him how she's feeling. But she runs into a crying Lavinia first. Lavinia is all worried about Matthew's safety as he's about to return to the war. Seeing her tears and knowing how much she cares for Matthew has made it impossible for Mary to break the couple up. It would seem that the Dowager Countess was the one who was right all along: "The truth is neither here nor there, it's the look of the thing that matters."
As for Lady Edith: How pathetic is she? She's always copying her prettier sisters. Last season she took a stab at Matthew when it seemed that it wouldn't work out with Mary, and now she's biting off of Sybil by trying out a tryst with a commoner. She's the worst. "Well, what about my dress!?" Ugh!
Lastly, it would seem that the war is going to have a much bigger impact on Downton than a lack of footmen. With the new epidemic of "shell shock," soldiers need somewhere to convalesce, and what better place than a giant, half-empty mansion?