This is a one-shot one-encounter game. The encounter will involve combat with a to-be-determined dragon. Combat will occur on an infinite featureless plain. In fact, the entire plane of existence is a single plain. Clear blue skies stretch to infinity above, and dirt stretches to infinity below, and short green grass stretches to infinity in every direction.
Should the heroes stomp the dragon with remarkable ease, another dragon shall appear, and perhaps a third, until such time that all the heroes are dead or the DM is satisfied that an adequate challenge has occurred (probably in the neighborhood of 10-15 rounds).