It's full of delicious ideas!
Or maybe your idea is simply to improve on something that you've already seen. Far from plagiarising, this is actually a fairly common fact of game design. Everyone can make games about soldiers and guns, but what if you were a firefighter and your gun was a firehose? Innovating and building upon existing ideas can be just as valid and important as (and often easier than) coming up with something completely new. The trick is to emphasise what makes this game unique, not what makes it just like everything else.
Remember when Lucy gets lost in snow-covered Narnia and has to find her way back to the street lamp that serves as the signpost back to the wardrobe? Or how about when Edward has to escape from the White Witch after she tricked him into coming back to her castle with the promises of power and turkish delight?
It's full of delicious power!
Once we've found a couple of cool story ideas you've got to stop and ask yourself a question. "It was exciting when I was reading about this, but how would it feel if this was really happening to me?" This is the key distinction between a book and a game. In a book, the reader is rarely a character in the book, rather they are a detached observer. In a game, the player is the star. They are front and centre when things go well and when disaster strikes.
When the player feels scared it's because there's scary things in front of them. When they feel victorious it's because they've struggled and succeeded against difficult puzzles and obstacles. Emotions are very personal for the player. How would you feel if you were lost in Narnia hunting for the lamppost? How would you feel if you had been tricked and kidnapped by the evil White Witch?
You want your player to feel exactly like that.