You must give them.
Mysteries are fun. They're compelling. They make for great stories. Something strange is happening, and it's up to our heroes to figure out what's going on and stop it.
As a writer, if you string your reader along with a mystery, you have a responsibility to explain to your readers what is going on.
Take MURDER ON THE ORIENT EXPRESS. Great, fun book. Agatha Christie presents us with a murder. She goes through the suspects. Then she tells us who the murderer is. (I'm being deliberately vague in case someone hasn't read it.) However, what if she had stopped there? Told us who the murderer was but not why the murder had been committed. That'd be okay, right? WRONG. The mystery isn't just WHO killed a man, but WHY.
I've been watching TORCHWOOD: MIRACLE DAY over the last ten weeks. I've been a fan of the DOCTOR WHO spinoff for some time. But I was blown away by the miniseries CHILDREN OF EARTH. It was a 5 part series that was a nearly perfect example of how to tell a story. The conceit was simple, the stakes insanely high, the pacing brutal, the explanation simple and clear, and the conclusion compelling. I had high hopes for the second series which involved a day when people stopped dying. It was a wonderfully high concept idea.
Writers, when your editor and agent talk about a high concept idea, this is what they're talking about. An idea that can be distilled to one line. Miracle Day is about what happens when people on Earth stop dying. You can imagine the implications. Easily understand it.
My problem with the show was that I spent ten weeks following this. I expected to find out who caused the miracle and why. If you don't want to know anything about it, stop reading now. I won't spoil any specific plot points, but be warned.
Last night the show concluded and we learned HOW the miracle came to pass, but the WHO was left vague. There was even a scene when one of the characters tried to explain what was happening and another looked at him and said, "You have no idea what this is, do you?" And they left it at that.
Was it alien? Was it terrestrial? Who know? And they're never going to tell us.
As an audience member, I was left feeling ridiculously unsatisfied. As a writer, I was embarrassed.
Now, I'm not saying that a story can't have a central conceit that goes unexplained. Hell, I did it in my own book. I asked the readers to accept that in my world, every person got a letter warning them 24 hours before their death. But I never made the book about trying to find and stop the letters. My book was about the implications of getting such a letter. In early draft, my characters did try to find the source of the letters. My agent and editor wisely helped me realize that no answer I gave would ever be satisfactory.
If Miracle Day had been about the world once people stopped dying, and focused only on that, the why or the who wouldn't have necessarily mattered. But that wasn't the case. This show was set up as a mystery. THING A happened. By the end of the series, we were promised that THING A would have an explanation. That promise was never fulfilled. Not only is that sloppy, but it's irresponsible.