Unless thatgamecompany (which is in fact the name of their game company) has something equally as charming up their sleeves you are not likely to see a game like Flower ever again. It is so simple yet so innovative in its simplicity.
You start the game with one flower pot in an apartment window and you begin the first level with one flower petal. It is never explicitly stated but your "avatar" is charged with brightening the world from the drab black and white that it has become. You will feel something warm grow deep inside of you as you play through this game. It can easily be played in one sitting and you would be foolish to consider this a caveat. I would recommend this for a first sitting with the game. Then come back after a week (nab yourself that trophy) and do it all again.
There is enough to go through the game at least a second time if not a third; you might find that you just want to be playing the game because of its beauty. The music is absolutely soothing and gratifying and your ability to subtlety control it adds to the experience.
There is not a lot more to say about the game other than it comes with the highest recommendation. It is simple, beautiful, and an experience you will not find anywhere else.