A summary, paired with the February 2017 release's chaste tease, attempts to shine light on the Darker situation:
When a wounded Christian Grey tries to entice a cautious Ana Steele back into his life, she demands a new arrangement before she will give him another chance. As the two begin to build trust and find stability, shadowy figures from Christian’s past start to circle the couple, determined to destroy their hopes for a future together.
Fifty Shades of Grey had a sense of humor and liveliness to its sex scenes. Fifty Shades Darker looks fifty shades more serious. There's a different team behind this movie than the first, which may explain the shift; Darker was written by E.L. James' husband and directed by James Foley (Glengarry Glen Ross, Fear). And if you're going to sequelize an erotic thriller, at some point it'll need more than the erotic to keep it up.