Hearts of Iron II: Doomsday contains everything that was in the original game while highlighting and expanding the period after historical World War II, including a hypothetical nuclear war between the Soviet Union and the Allies. The expansion pack features among other things a reworked intelligence model (which allows the player to use espionage, sabotage and other things in an "intelligence page" accessible through the main screen), improved AI and a scenario editor.

Doomsday also features some other changes and additions such as:

*Additional graphics such as bomber sprites
*Escort Carriers
*Hospitals that recover manpower losses
*A "do not upgrade" button for divisions
*The option of automating the production sliders
*Submarines have separate stats for convoy raiding and naval combat
*An extended timeframe — the game end date has been changed to 1953
*New technologies to bring the game into the cold war era
*An included scenario editor
*The ability to trade divisions between nations
*The ability to attach escorts while recruiting divisions