Alice asked in the comments a few days ago about creating a logline for a romance novel, inparticular as Save the Cat! mentions that you need an adjective for a villian. Many category romance novels do not really have a true villian. The emphasis is on the growth of the emotional relationship.
HOWEVER, there is always a conflict between the hero and heroine. And you can show this in a few words.
For example with Viking Warrior, Unwilling Wife, the logline might be -- To save her son, a strong willed woman must join forces with the one warrior most capable of destroying them both -- her former husband.
So you want to show that there is going to be a conflict worth sustaining, and that the book fits the genre you are writing in. It is a matter of knowing your hooks.
The logline in romance is really the middle paragraph of your query letter. You do have more than a single sentence.But if you can make it nice and pithy, you do give the editor somewhere to hang her hat.
I hope this helps.