Solarium gives Twine a good name. This well-crafted game is adult ficion, not as in sexuality, but as in dealing with thoughtful and meaningful concepts. It involves alchemy and an alternative ending to the cold war, decades ago.
The narrative has a branching structure, with each branch requiring a key in the form of an alchemical substance. By obtaining more substances, you unlock more areas.
The game includes several striking images, including scans of real government documents.
I strongly recommend this game.