"Doom" is a shareware game of three episodes, each having eight standard levels and one secret level. The secret level in each episode is accessed by finding a secret exit in one of the normal levels. The episodes are "Knee-Deep In The Dead", "The Shores Of Hell" and "Inferno". In the shareware version the only episode that can be played is "Knee-Deep In The Dead". The expansion pack of "Doom" is "Ultimate Doom", which has the fourth episode, "Thy Flesh Consumed".