Normal sudoku rules apply. Each of the digits 1-9 appears as a „Doubler“ exactly once in the grid. There is only one „Doubler“ in each row, column and 3x3 box. Digits that appear as a „Doubler“ have a value that is double their size. The value of digits in a cage must sum to the small clue in the top left corner of the cage. Digits may not repeat within a cage, although values may. The value of digits along an arrow must sum to the value of the digit in the attached circle. Each arrow has its own colour. Arrows of the same colour are connected. Additionally the grid is partially covered in fog. Placing correct digits will clear the fog from surrounding cells, possibly revealing more clues.