Certain movies are hard to classify in the star rating system. Downton Abbey is one such film. Fans of the TV show will likely enjoy what is essentially an extended episode of the series. But if viewed as a stand-alone movie, it doesn't entirely work.
At the start of the film, Lord Grantham (Hugh Bonneville) receives notice that the royal family will be visiting Downton Abbey. The Grantham family and their loyal servants must prepare while dealing with assorted personal dramas. There's too many characters for me to adequately summarize all of these dramas in a simple review.
On the one hand, fans of the series will have fun spending time with the show's characters once again (full disclosure: I have seen every episode of the TV show and mostly enjoyed it). Maggie Smith, as the family matriarch, gets to deliver plenty of funny zingers. The costuming, set design, and acting are all good.
However, and perhaps this is only a concern to curmudgeonly critics like myself, if you are going to release a movie in theaters and not play it on PBS as a special episode of the show, it needs to function as a stand-alone movie. Downton Abbey doesn't. It doesn't pick a character to focus on, so the narrative feels a bit all-over-the-place at times (as a counterpoint, the 2007 The Simpsons Movie wisely made it Homer's story). And because there is no main character arc in the movie, it doesn't quite know when to end and it feels like it runs on for maybe 20 minutes too long.
Ultimately, it's a film that will probably please its target audience, but it's unlikely to make any viewers want to go back and discover the show.