Space to start. Pieces fall, you rotate and slide them, complete rows disappear and the ones above drop down. Score in the corner, row count under it, and the next piece where it should be. Every key does what you expect on the first try.
There is no menu, no mode select and no music. The speed climbs with the row count and does not stop climbing, so a run ends the way runs of this game always end: with a piece you needed three seconds ago and a stack that has been leaning left for a while. It is the plainest possible version, which is the version most people actually want.