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The dog bit was all about Thomas the footman wanting to impress Lord Grantham to get promotion. Thomas hides the Lord's dog Isis in a shed in the woods, with the aim that the Lord misses Isis madly, everybody searches for her, and Thomas then "finds" her. His plan was scuppered when, on going to the shed to retrieve Isis, she'd gone. He frantically searched for her and gets his clothes in a mess while doing so. When h arrives back to Downton empty-handed, he bumps into the Lord happily reunited with Isis. Lord asks Thomas why he's so dishevelled, Thomas claims it's cos he was searching in the woods for Isis, Lord is grateful and impressed and later seeks promotion for Thomas - so Thomas' plan looks to have worked anyway. However, if Bates is set free, Thomas will presumably return to his current post.