Mystery cards represent special story elements. They are only drawn when instructed by a game effect, such as an Adventure card or part of the special rules for the scenario.
There are three types of Mystery cards: Traps, Treasures and Creatures. When you are instructed to draw, the instruction will always tell you which type(s) to resolve and how many. Turn over cards from the Mystery deck until you find a card of one of these types, then draw and resolve it as follows:
Treasures go in the Future Resources space.
Traps and Creatures must be resolved immediately, although you may use other abilities to influence the outcome (e.g. special abilities, tokens, previously acquired Treasures).
After resolving a card, you may either choose to stop or continue. If you continue, repeat the above process, turning over more cards until you find another that matches the specified type(s). If you stop, you do not have to turn over any more cards. If you have reached the limit of cards you were told to draw and resolve, then you must stop. (Cards you have turned over but not resolved because they don’t match the type(s) don't count as ‘drawn’.)
When you stop, either because you choose to do so or because you reached the limit for all type(s), shuffle back into the deck all the cards you turned over but didn’t resolve, and discard the ones you did resolve.
If the Mystery deck ever becomes empty, shuffle the discarded cards to form a new deck.