The United States Bullion Repository, housed next to the massive Fort Knox army base in Kentucky and occasionally referred to by the same name, is a concrete and steel fortress stacked with over 5,000 tons of gold bullion; for a wily bank robber armed with the latest electronic gizmos it's a smorgasbord of bling just waiting for someone with a large enough plate. Of course, no government in their right mind would leave all this shiny stuff lying about the place unguarded and what appear to be giant rats have therefore been released into the repository corridors to take care of unexpected visitors.
The player is a well-equipped and presumably brave thief whose objective is to navigate the vaults to collect the gold bars one at a time before returning to load their waiting van, all of which must be done whilst steering clear of rat-like guardians. Grabbing a bar starts a timer in the status panel going that on expiration will cause the enemies to speed up until either the loot is safely stowed or the criminal is captured. The critters themselves aren't particularly bright and, rather than actually looking for intruders to capture, rely instead on wandering around the complex and bumping into them before presumably strip searching for cheese.
IT'S A RAT TRAP AND YOU'VE BEEN CAUGHT
Return to Fort Knox is an unofficial fan-made sequel to Commodore's 1982 cartridge release Raid on Fort Knox, based on the original code but expanding on the gameplay by bolting in enhancements like an increased number of enemies in play, extending the number of unique vault layouts to eight and offering a freeze ray for the bandit that can temporarily stop roaming rodents in their tracks.
Attempting to enhance an existing title is a risky undertaking and the results can, as with some film sequels, produce something that is merely a pale imitation of the original, but developer Nino Porcino has done pretty much everything right and improved on what was already a solid game without drifting too far away from what made it good to start with. The difficulty curve is pretty steep and not everybody will find it enjoyable, but Return to Fort Knox is simple enough to get into initially whilst being significantly harder to actually master.