Four contestants stood within a large, transparent plastic cube, one at each corner. They faced the host, who stood next to a safe and a tray of 19 numbered keys in the center. Only one key would open the safe. Before each contestant was a panel displaying the key numbers. A cash jackpot was at stake, starting at zero and increasing at a continuous rate of £500 per minute for 15 minutes. If the jackpot reached its maximum of £7,500, it then began to decrease at a rate of £2,500 per minute for the next 3 minutes until it reached zero again. A game could thus last no more than 18 minutes. Once the timer and money counter began to run, they did not stop except during commercial breaks. Throughout the game, a contestant could eliminate wrong keys from their own panel by answering questions correctly, while a miss would relight them. The specific keys to be eliminated/relit were revealed only after an answer had been given. Once a contestant had eliminated a total of seven keys, they could relight either two keys on one opponent's panel, or one key each for two opponents. Each contestant knew only how many keys their opponents had eliminated, but not specifically which ones. Round 1 consisted of quick-fire questions on the buzzer, open to all contestants. In Round 2, the questions came in pairs; the first was open to all, and the contestant who answered it correctly could either attempt the second one or pass it to an opponent. One key was at stake for each question in these two rounds. In Round 3, each contestant had 30 seconds to answer as many quick-fire questions as possible. On each question, they could choose to play for one, two, or three keys. Round 4 used pairs of questions as in Round 2. Now, though, the contestant who answered the first one correctly could set the difficulty level for the second, which was again open for anyone to answer. At any time, a contestant could press a red button on their podium. They then had 10 seconds to select a key and try to open the safe with it, with the timer and money counter still running. If successful, they won the entire jackpot; if not, they were eliminated from the game. If the jackpot decreased all the way to zero before the safe was opened, all four contestants left with nothing. A special episode with game show hosts featured Jeremy Beadle, Henry Kelly, Nick Weir and Nicholas Parsons.
