A man with no memory is trapped in a mysterious hotel, controlled by an oppressive host who forces him to to catch a killer among the guests in exchange for the truth of his identity.

How do they know their memories are lost? I mean, how can anyone know they lost memories, that they need to get them back, if they can't remember? (The central conceit of the "Memento" is that the protagonist could form no new memories after being shot -- which happens to be medically factual. So the conceit is believable.) I hate to keep hammering on the point, but I, for one, am having difficulty suspending disbelief.

And why is it central to your story that they're lost their memories? Being trapped in a surreal version of the Hotel California ("you can check out any time, but you can never leave") would seem to be a sufficient dramatic problem in itself.

A revision:
A young couple must solve the mystery of their imprisonment in a surreal hotel masquerade ball to recover their lost memories.

Yes, you've summed up the challenges very well.

The problem with sci-fi and fantasy genres is that they entail a suspension or revision of one or more of the rules of the normal world; ergo, while the world of your story makes perfect sense to you the creator that world, it's completely terra incognita to the rest of us; it doesn't make immediate sense.

So, for example, your logline says that your protagonist has "no memory". Well, then how can he possibly possess the knowledge and skills to do whatever he's supposed to do?

Why would the "oppressive host" employ the character to "do his bidding" when he is crippled with such a debilitating, cognitive deficit?

sci-fi mystery. A "Twilight Zone" story.

What's the genre of the story?

What's the genre of story?

The bidding is the "catch a killer" part of the original log line, but I'm starting to think it gives the wrong impression.

"Defiantly curious" or "defiant and curious"?

What's the bidding? What's the specific objective goal he must achieve to get his memories back?