Arrows and WASD to move. Space or click to shoot. If you touch the ball, it is game over and you have to click the green flag to start again. If you click the green flag and it does not work, move around and shoot and it should move the ball.
Credit to @griffpatch for the plane/ship. Also, this is just a remake of Lemon Invasion!. (not a typo) I think it's pretty good!