Start the game by selecting which game mode you want. You can opt for local multiplayer or P2P. If you choose to play P2P, you need to select your opponent from the peer list.
Ultimate Tic Tac Toe is an alternative version of Tic Tac Toe, in which strategy plays a more significant role. It's Tic Tac Toe within Tic Tac Toe.
You play on nine (9, yeah) Tic Tac Toe boards simultaneously, and the results of those boards contribute to a much larger board. When you complete one mini-board, that mini-board cannot be changed anymore, and the result stays on the large board.
But here's the caveat. The cell you select on the mini-board determines the next mini-board you are playing on! It means that sometimes you will sacrifice one mini-board for the sake of the large board.
The game starts by allowing the first player to select the mini-board to play on. Once the first player chooses a cell within the mini-board, the mechanism to restrict the mini-boards starts to apply.
As the game progresses, more cells from the mini-boards will fill up. As this happens, you will face the following scenario.
The mini-board you are faced to play on already has a winner. In this case, you can select which mini-board to play on next.
It happens when you select a cell that points to another mini-board or the same mini-board you are currently on.
If you can select the mini-board to play next, you can change your choice before clicking on one of the cells.
This is a homage to the original Ultimate Tic Tac Toe. I used to play this game when I was 10 years old, so it holds some special meaning.
You can find the entire source code (and infrastructure code) in GitHub.