Normal sudoku rules apply. Every digit that appears in a circle must also appear in at least one of its 4 surrounding cells. If an arrow is present in a cell, the digit in the cell the arrow comes from appears in the grid in the direction of the arrow at a distance of N cells, where N is the digit in the arrow's cell. (Not all arrows are necessarily given - there is not negative constraint.) Example: If R3C4 had an arrow pointing to the left and was a 3, and the digit below that arrow in R4C4 was a 5, then another 5 would be placed in R3C1. The grid is partially covered in fog. Placing correct digits will clear the fog from surrounding cells, possibly revealing more clues.