The Princess of Wales is taking another step in her return to public life following cancer treatment as she hosts her annual Christmas carol service at Westminster Abbey.

Nick Briggs/Carnival Film and TV/Masterpiece/PBS(CNN) - "Downton Abbey" reopens its grand doors, in a movie where half the battle -- and much of the thrill -- hinges on merely reassembling the enormous cast. After six seasons that yielded the kind of ratings PBS seldom sees, "Downton" closed its run in 2016, with happy endings all around. It's easy to argue that this trip back was unnecessary, a description that applies to many a revival and reboot. Ultimately, though, it's another satisfying chapter, demonstrating that enough still glitters around "Downton Abbey" to make it worth seeing for anyone who loved the show, eventually, if not sooner. "Downton Abbey" premieres Sept. 20 in the US.