There are strange things going on in your basement laboratories. They keep calling up for more barrels of quicksilver, or bits of your hair. Well it's all in the name of progress. They're looking for a way to turn lead into gold, or at least into something better than lead. That lead had just been too good of a bargain to pass up; you didn't think, where will I put all this lead, what am I going to do with this lead anyway. Well that will all be sorted out. They're also looking for a universal solvent. If they manage that one, you will take whatever they use to hold it in and build a castle out of it. A castle that can't be dissolved! Now that's progress.
This is the 3rd addition to Dominion. It adds new Kingdom cards to Dominion and as it is an expansion, you will need Dominion, or Dominion: Intrigue to play the game.
Dominion: Alchemy is an expansion, and can't be played by itself; to play with it, you need Dominion, or a standalone expansion to Dominion (Dominion: Intrigue). Those provide the Basic cards you need to play (Treasure, Victory, and Curse cards), as well as the full rules for setup and gameplay. Dominion: Alchemy can also be combined with any other Dominion expansions you have.