I'm not entirely sure as I haven't read it specifically. But generally, it might be about the experiences of lighthouse keepers during Christmas. It could involve how they celebrate the holiday in the isolation of the lighthouse, perhaps with some special traditions or events that make their Christmas unique.
The main characters are most likely the lighthouse keepers. They are the ones who live in the lighthouse and experience the Christmas in that unique setting. There may be an older, more experienced keeper and a younger one. The older one could be passing on the Christmas traditions of the lighthouse to the younger one. There might also be a character who is in charge of supplies and how they manage to get something special for Christmas.
Not entirely. 'The Keepers' likely takes inspiration from real incidents but incorporates fictional aspects to make the story more engaging and compelling.