A grid, a black hole, and a handful of blocks. Pick a direction and the whole board slides that way until something stops it, which means every move commits the entire arrangement at once and there is no nudging a single piece into place. Blocks that reach the hole are gone. Blocks that reach a wall stay where the wall put them.
Neutron stars, one way arrows and a few other pieces arrive over the next dozen levels, each of them a rule that changes what a slide means rather than a new button to press. The presentation is quiet almost to the point of being austere, and the puzzles are the sort where the answer is obvious for about eight seconds after you find it and invisible for the twenty minutes before.