Meet my Hatchet of Death (or, some other colorful description RC Lewis and I come up with at any given moment). This is how I edit myself, it is how I edit others. If you think you want to play with me and my hatchet, shoot us an email.
We all know the first line of a query is your "hook." I call the last line the "sinker." You want it to punch them in the face, in a nice, friendly kind of way that makes them unable to forget you after having read the 300 other queries in their inbox.
If you're looking for query advice, but are slightly intimidated by my claws, blade, or just my rolling googly-eyes, check out the query critique boards over at AgentQueryConnect. This is where I got my start, with advice from people smarter than me. Don't be afraid to ask for help with the most critical first step of your writing journey - the query. My comments appear in green.
Kelsey thinks déjà vu is normal. Just like everyone else. Slightly confusing here - but only a bit - as in, everyone else also thinks it's norma, or she thinks everyone else has it, too?It's a tiny, tiny nitpick, on my part.
But she’s wrong. Her déjà vu is the sign of a terrible problem. Hmm... deja vu definitely has me interested, but a terrible problem and a possible paranormal link isn't that original in the query box of YA agents. What about yours makes it unique? Get it in the hook!
As soon as Kelsey turns 18 she will be shot and killed by two assassins from the future. But there’s one glitch—one of the assassins can’t be born if Kelsey dies. A paradox is created and time loops. Until a man named James contacts Kelsey. Man, or boy? What's his age? Very interesting. I'm a time-travel fan so this has my attention, but the first thing that makes me go "Huh?" is - why is an assassin coming to kill and 18 year old girl? If it's because she's going to give birth to someone important, then this is straying pretty deeply into Terminator territory. If it's because she herself is the target, I need to know why in order to believe that there's an inter-time hit out on her.
Claiming to be from the future, James explains the loop to Kelsey and says he has been sent to save her and resolve the paradox. Despite her many doubts, wait - wouldn't it be to her benefit to stop the assassins anyway? I mean... Kelsey cooperates with James to stop the assassins. But soon it becomes clear that James is hiding something from her: His real mission. Well, right... because if he DOES save her, then the loop is closed, her assassin is born... and... she dies? Gah - OK you definitely have my attention.
WHEN TIME ENDS is a 73,000 word YA science-fiction thriller.
I think this could work if you get your essential problem into the hook- she needs to live in order to die, right? Get that out there, front and center! You've got an interesting plot here, but make the hook work for you and clear up the question of why her death needs to happen in the first place (beyond the space-time continuum issue). Lastly, is there a romantic angle with James and Kelsey? If so, make that clear.