While following the leads after her missing brother, Rita arrives at mountain chalet. But instead of her brother, she finds a secret underground laboratory, full of horrific creatures trying to eat her.

That attempt is a lot better, If you can find a way to do that in one sentence I think you'll have it.

Thank you Richev. Every advice pushes me a little bit more forward:)

How about this?

While investigating her brothers disappearance, young pathologist Rita discovers a secret underground lab filled with horrific man-eating creatures. In order to survive, she must confront the monsters, and even her own brother.

Oops I meant "tenacious young woman," not a "tenuous young woman," hehe just re-read and caught that.