There actually are people who've never seen either the TV series or the first movie. This film has Easter eggs to delight the fan base but new viewers will be confused. For instance:
These developments only make real sense if you know the backstory for each character.
- Edith and Bertie name their baby Peter
- Lady Grantham develops pernicious anemia
- Lady Mary declines to begin a romance
- The Dowager says she lost her head over Prince Kuragin
- Carson's hands seem just fine
- Andy is put forward as the new butler
These developments only make real sense if you know the backstory for each character.