The year is 2117. Climate change has taken its toll on Earth and new resources are needed to fuel ever growing megacities around the globe. Research during the last 50 years has shown that the Moon has several very pure and effective mineral resources that are needed on Earth. This year, mining corporations have established bases on the Moon with the purpose of building a transportation network to mine valuable mineral resources. As these resources are of global interest, the top 20 nations on Earth have invested in a freight rocket, “Future One”, scheduled to fly to the moon and transport minerals back to Earth.

In 21Moon, players corporations have eleven months to gather as many minerals as possible before the rocket leaves the moon. Players are opportunistic investors who see an excellent opportunity to make credits (=money) by investing in and running private companies and mining corporations on the moon. The corporations establish bases on the moon and build road networks to valuable mining resources, mining as much as possible until the freight rocket leaves the moon with its cargo of minerals.

The winner is the wealthiest investor when the rocket leaves. An investor's wealth is made up of personal credits and current market value of owned shares in the seven corporations.

21Moon is an 18xx game.