The Mayor and I played this after finishing one of the dICEHOUSES. We were both surprised at how fun this descendant of Battleship was. In a series of specific location probes you are able to triangulate the location of your opponent's spies. The name of the game comes from the rare occurrence when your opponent's spy is on the same location as your spy. By announcing "Checkpoint Danger!" you are revealing your spy's location in an effort to capture their spy first.
If it sounds complicated, try playing it after a few (d)Icehouse Ales.