Tasty Planet is game focused on the evolution of a prototype bathroom cleaner, "Grey Goo." The levels follow the Grey Goo as it changes from eating dirt particles to bugs and leaves, and then eventually the rest of the universe. The game is arcade in style, and has both a casual mode, where players can play for leisure without a timer, and the full game which is timed and must be completed before the time runs out to complete the level. There are also 3 bonus levels called Endurance levels, which are incredibly long levels, and can take up to an hour to complete each.
The plot is shown solely through comic strips at the end of some levels. The first comic strip shows a scientist telling his assistant about his latest discovery, the Grey Goo bathroom cleaner. They place it under a microscope, and the first level of the game begins. After the first two levels of the game the assistant touches the Grey Goo, who bites him and enters his body. The scientist tells his assistant to "Just go and wash your hands."
The Grey Goo is then washed down the drain and lands outside, where it moves to a park and grows in size. It then moves on to a picnic table where the scientist and assistant are having lunch. When discovered, the pair throw the Grey Goo into the ocean. The Grey Goo then eats through the ocean and is launched by a whale into another park and then to a city. There, the Grey Goo face its first opposition from humans, who attempt to use guns, tanks, etc. to stop it. It eventually launches itself into the sky and then into orbit around the Earth. After eating the moon, Earth, and the rest of the solar system, it moves on to the Cosmos. An alien reports as missing in a distant galaxy, and soon the Grey Goo eats through galaxies and superclusters before eating "The Fabric Of Space," and "Time Itself." After eating the entire universe, a short cut scene follows. The comic strip describes how the Grey Goo's mass was too great to sustain, and it imploded, causing the universe to start again.
The main objective of the game is to get to the size specified by the level. This is done by eating entities that are smaller than the Grey Goo, and avoiding those that would harm or eat it, such as police officers, amoebas and white blood cells. The level is complete when the Grey Goo has reached the required size. On the timed levels, players can also try to achieve the "Medals" for each level by completing the level under a certain time limit, which can be found on the level select menu by mousing over the chosen level.
Tasty Planet is split up into 9 chapters, each containing a number of levels. They are listed in increasing size order as the Grey Goo grows.
An additional option selectable from the main menu is the Endurance mode. These are variations on 3 existing levels. The only change is that you grow extremely slowly, and it can take over an hour to beat one of these levels. They are considered the most difficult part of the game.