On what simple ironical reversal is the plot of "The Most Dangerous Game" based?
The title of "The Most Dangerous Game" refers to the "game" that General Zaroff has created. I put the game in quotations because it really isn't a game - it is a savage hunting of a human being. But as Rainsford himself says, Zaroff is "like all his race, a bit of a savage". The "game" is the most dangerous because it usually ends in death for the "hunted" player - games themselves are not usually fatal.
However, the title shifts direction in the end, and this is the ironic reversal of the plot. While Zaroff has set up Rainsford to be the hunted, and therefore the one in danger, he underestimates his prey. Rainsford is able to elude Zaroff, mostly because of Zaroff's arrogance. Zaroff is able to catch up to Rainsford, but because Zaroff is too into his "game", he lets Rainsford get away so that the chase can continue:
Rainsford's second thought was even more terrible. It sent a shudder of cold horror through his whole being. Why had the general smiled? Why had he turned back?
Rainsford did not want to believe what his reason told him was true, but the truth was as evident as the sun that had by now pushed through the morning mists. The general was playing with him! The general was saving him for another day's sport! The Cossack was the cat; he was the mouse. Then it was that Rainsford knew the full meaning of terror.
Unfortunately, this is Zaroff's mistake and leads to the reversal. Rainsford does get away, back tracks, and is able to find Zaroff in his own bedroom and kill him. Thus, the "game" has become the most dangerous for Zaroff himself, the supposed hunter.