Carrying over data from scene to scene?

I'm working on a small prototype adventure game. It's nothing fancy, I just want to try and get some of the core mechanics of an adventure game down inside of Unity.

One thing I'm most curious about is how I can tell the engine to recognize if the player has had a conversation with a game character that takes place in a scene.

For example, I have a character in scene 'A' that has a key for a door that is in scene 'B'. After talking to the character in scene 'A' I get the key. How can I carry that information over to a different scene? So the door knows I actually have the key by way of getting it from a different scene?

The same for anything else, not just a key. A conversation, a plot point etc. How to check if the player has seen that information before proceeding on with the story.

I hope that makes sense =\

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