Basically, its a tic tac toe game, but slightly different.
In the beginning, all "O" or "X" had been decided and set behind the scene, hidden by the blue square button.
In each of 9 big squares of the tic-tac-toe, there are 4 sub-squares hiding 2 "X" and 2 "O" behind the scene.
The player will flip a sub-square in each turn, it may be "O" or be "X", which means an O-player may flip an "X" out in its turn as well, and vice versa.
If a player wants to draw an "O" in a square, he needs to flip out the 2 "O" at first, and vice versa.
As you can see, the first player may not have the advantage, and the second player may still have the chance!
Thats the fun part of this Tic-Tac-Toe(Dark)!
Hope you have a great time with your friend playing with it!