Some of the Butterflies have chemical defense against certain predators. In the caterpillar stage, they consume certain plants and store the chemicals which prevent some predators to eat them. These predators, specially birds are known to remember the distaste and associated wing patterns. To take advantage of this, certain species copy the pattern to avoid the predators. This is called as Batesian Mimicry. Another group, which is also distasteful copies the pattern to make sure the predators remember the pattern, these are called Mullerian mimics.
Plain Tiger or African Monarch (Danaus chrysippus) feeds on Milkweed plants in caterpillar stage so has the required chemical defense making it a model in this mimicry.
One of the very common mimic of this model is Danaid Eggfly (Hypolimnas misippus) in which only the female mimics the Plain Tiger butterfly and male is completely different, so much so that one might think it is a different species.