The story of the game is that a dragon warlord known as 'Basso Gila' teleported his castle and army to the peaceful animal kingdom of 'Moberry' in order to destroy it and establish his ultimate rule. The council of elders of Moberry put Charly the highland alligator on the task to send the invaders packing to unsure their well deserved freedom.
There are five areas in Tail 'Gator, each of them divided into four sections: Air, Land, Cave and Underwater, except for the fourth and fifth areas which are only Castle.
The player must complete all sections in order to advance to the next area, this is done by collecting all treasure chests in each level, they all contain random items except for the last chest which is always a key to unlock the door.
Each section has four levels, in area 2 section Land and area 3 section Underwater there are secret levels which are accessed by a hidden door that's partially revealed by obtaining the key. The fourth and fifth areas only have three levels.
Source: Wikipedia, "Tail 'Gator", available under the CC-BY-SA License.