The story in Only Love is Deathless jumps into the action right away, with a prince coming to an agreement with a thief that they will get married, but it will be a fake marriage and only for a year. This will allow the prince's younger brother to marry his true love, because the rules of the land dictate that a younger member of royalty cannot marry until their older sibling does.
From the start, nothing goes as planned. The fake dragon the thief (who has magic) was supposed to conjure up so he and the prince could "meet" by dramatically battling the beast together, is actually a real one sent by someone unknown (but clearly evil), and the thief gets badly injured. Cue the prince and the thief pretty adorably falling in love with each other while the thief convalesces.
From there, the story essentially follows this format. Sweet love story, lots of battles with mythical creatures, a quest, and some highly dramatic tragedies. Just a note that there are quite a few explicit "strong sexual content" (as the movie rating system would say) scenes in this book. I know some people prefer to skip the erotica, so just a heads up there. The scenes start out well-written, demonstrating the love between the two characters, but then there are so many of those scenes, and they are written so similarly, that it just becomes very repetitive. Some variety in writing style would have helped there I think. But it's an enjoyable read with some sweet romance and a lot of page-turning drama, and flew by pretty quickly.