In Reus, you control powerful giants that help you shape the planet to your will. You can create mountains and oceans, forests and more. Enrich your planet with plants, minerals and animal life. There is only one thing on the planet that you do not control: mankind, with all their virtues and and all their vices. You can shape their world, but not their will. Provide for them and they may thrive. Give them too much and their greed may get the upper hand.
Reus is about shaping nature and about the greed of mankind. The player controls mythical giants, through which the player controls every aspect of nature. One can perform feats like terrascaping, creating life and genetic manipulation. However, eventually humanity enters a stage where they escape from the control of the giants. Change can still be made to their environment or oceans can be made underneath their cities, but their will is their own. The challenge is to let mankind and nature live in harmony, while encountering the many vices humanity has to offer. This means that as a player, some of the tasks are solving human wars, countering deforestation and even surviving attacks against the giants themselves.