Based in H.P Lovecraft's Cthulhu Mythos, you play an investigator (a double sized card that has your stats, special abilities and keeps your story organized) who must complete stories without going insane. From a deck of 52 cards you draw 13 and take turns playing one at a time so it moves at a brisk pace. You stock your deck with at least 20 points of stories (usually 3 or 4) and try to get into play the required cards. As an example The card 'A Day in the Life of a M. U. Student' requires you to visit 3 Different M. U. Locations, cast a spell, summon a monster, go to a country site and play an ally of the opposite sex. When you have fulfilled the conditions you score the card and gain sanity. In this cards case you score 8 points and gain 2 sanity.
The types of cards you have are locations (of which you are always in one), allies, monsters, events, tomes, spells, artifacts and adventures. A round lasts until there are two passes at which time all monsters played fight each other then go after your investigator to reduce his sanity. You can throw allies in front of the monsters as cannon fodder to protect your investigator. You then decide what cards you are going to keep in your hand (each investigator has a min and a max amount of cards that must be kept ) and draw back to 13 for the next round. Because of this you blow through your deck very fast and you will go through it a couple of times in most games.
There are also some very nasty phobias and events to play on your opponents to mess with them. There is a lot of interaction and the Lovecraft type atmosphere is very high.
Some say that Mythos is a difficult game to play from a starter set as there are specific cards you need to score with. But, collecting is very easy as the cards can usually be found cheap and there are two rares in each pack. - http://www.boardgamegeek.comChaosium's Official Mythos Card Game Web Page