Bertie and Robin are playing a game of Box, with the colors red and blue. Bertie goes first, playing red:
Robin decides to claim red (and the first move), and become the first player. Bertie therefore becomes blue, and plays the second move, taking the center. Robin follows up with the third move, as red.
Bertie plays defensively, and Robin attacks on the left edge:
Bertie threatens to make a 1 x 3 box. This appears to have been a mistake, as Robin simultaneously blocks it, and threatens two different boxes at once, for 5 (on the left) or 3 (on the right):
Bertie blocks Robin's box for 3, and simultaneously makes a threat for 6. Robin takes the trade, and shades a red box for 5 points:
Bertie claims a 2 x 3 box, and is up by one. (Note that Robin's red dot on the edge of Bertie's blue box does not block it.) Robin maintains the initiative by playing on the bottom and threatening to claim a huge box for 35 points....
... which Bertie, of course, blocks. Robin takes the opportunity to make a threat which cannot be blocked (because essentially the same 1-wide box can be completed at two different points):
Bertie counters this with a triple-threat, forcing Robin's hand; Robin scores 7 more points. (Note two things here: Robin blocked one of Bertie's threats by claiming a box which covers part of the bigger area Bertie was threatening to take; also, Robin's box overlaps the previous red box on the field (which is perfectly valid, though claming the same grid square again doesn't score another point).
Bertie happily takes 12 points. Robin counters with a move on the right, setting up to attack the top of the board (while also handily blocking one of Bertie's opportunities on the right).
And the game continues...
... until this point, where Robin passes, and then Bertie passes, ending the game. Note that there are still 4 or 5 grid squares that could theoretically be scored (if the other player was really not paying attention!), but both players believe that any possibility for scoring that they might have will trivially be blocked.
As occasionally happens, it turns out that they both missed something -- Robin could have actually scored one more point, by making the proper play (and Bertie should have blocked it). Can you find it?