There exists a saying in Germany which translates to "buying a cat in a sack". This saying "Die Katze im Sack kaufen" finds its equal in the English proverb "to buy a pig in a poke", and it is exactly this kind of uncertain bargain which has found its way into the new game Filou - Die Katze im Sack by Friedemann Friese. Thus, Filou literally is a game about buying Cats, and during the game the players will try to obtain the most valuable cats by participating in bidding rounds.

At the beginning, a deck of playing cards will be shuffled, and this deck contains mostly Cats with values ranging from "+15" to "-8", among with a few big and small Dogs and some worthless Pink Rabbits. Each player receives a hand of cards from the deck plus 15 coins (or 'Mice' as they are called in the game), and during a turn of the game each player will have to chose one of the cards from his hand which he places facedown at the auction area (the "Sack") at the middle of the table. The auction area is divided by a few cards, with one of these cards showing the proverbial sack and the other cards showing different amounts of cash. The first player places a card from his hand next to the sack, whereas each of the following players assigns his card to the next following cash card. The cash cards have been arranged in rising order, and at the beginning of the game an amount of cash equal to the sum shown on the card had been placed onto each card.

Once all players have played their cards, a bidding round will begin in which the players will bid for all the cards that had been placed for auction this round. Before the first bid is given, the first card in the auction area (the card next to the sack) is revealed so that the players know one of the cards which will be available in this auction. Each player then either may bid or fold, and with each player who opts to fold one additional card from the auction area will be revealed. However, folding also gives a player access to additional money, since each player who folds will receive the Mice which from the lowest still available cash cards in the auction area. Thus, the longer a player participates in an auction the more Mice he hay get. However, there always is the risk of all other players chosing to fold, and in this case the wagering player will not receive Mice but instead has to pay his bid to the bank and possibly may get a rather bad catch of cards.

When the auction is over, the winning player examines the cards he has won. If two or more Dogs were in the Sack they cancel each other out, but if only one dock was in the Sack it will depend on the size of the Dog what he will do. So, a big Dog will scare away the Cat with the highest value, whereas a small Dog will scare away the Cat with the lowest value. The player then takes the remaining cards to his personal pile of won cards, and a new round of bidding starts with the last winner becoming the new start player. The game then continues until a certain number of round has been played and the players have used up their hands, and then the players add up the values of the positive Cats, adding the value of their remaining Mice and substracting the values of their negative Cats. The player with the highest score will have won the game.

Especially the rule that the remaining Mice will count for victory might be underestimated quite a bit during the first rounds of the game. Thus, it is quite attractive to remain in an auction long enough to gain a high amount of Mice, but at the same time the players must make a guess whether other players might choose to leave the auction. This guesswork is made even more difficult by the fact that the players are allowed to keep their current amount of Mice secret, so players will need a good memory to keep the risk calculable. However, the fact that the cards in the sack have been placed faced down keeps a certain amount of luck in the game concerning the final value of the sack, and so the Friedemann Friese retains a degree of uncertainty which is very important for a game belonging to the category of small entertaining multiplayer cardgames.