Once upon a time in Vrindavan, Lord Krishna and his friends were happily playing about and causing a lot of mischief. Lord Krishna covered himself up in a cloth and tried to drink milk directly from a cow by pretending to be her calf. But once the cow saw it was a human, she lashed her tail at Lord Krishna and chased him away. Seeing this, the friends all started to laugh together loudly.
Now, there was a part of the forest where no one usually ventured. It was called Thalavana. But, Lord Krishna and his friends along with Balarama decided to visit the Thalavana that day. All of them went in and roamed around together until they found a tree fully laden with mangoes. All of them got hungry at once upon seeing the delicious mangoes. The mighty Balarama walked upto the tree and started to shake the tree with all his strength. Immediately, fruits started to fall over the place and as they fell on the ground, they made loud thuds.
Hearing this, a demon named Denukasur was awoken. He was a demon in the form of a big donkey who roamed Thalavana. Denukasur at once rushed towards Balarama at a high rate of speed and kicked him on the chest with his hind legs. Balarama fell down, but was not at all injured. He got up, but he was kicked again and he fell down to the ground.
This time, Balarama got up and swiftly dodged the kick of Denukasur. He instead caught hold of its hind legs and started swinging him around at a high rate of speed. Balarama swung Denukasur hard and let go. Denukasur went flying and crashed into a tree, instantly dying. Lord Krishna and the other friends praised Balarama for his feat and they went home happily.