Draupadi is born from fire, and has to be married. Her father Drupad and his son arrange a Swayamvar -- a challenge in which all are given a task and who so ever fulfills it; marries the girl. Swayamvar of Draupadi is arranged and all the kings and respected people are invited. The invites include Duryodhana, Krishna, and Pandavas etc. In this challenge, there is a rotating disc with holes and inside is a fish. The person who wants to marry Draupadi must seize the heavy Bow of Shiva, then look in the reflection and pierce the eye of the fish from the rotating disc.
Most of the kings including Duryodhana, Jarasandh, and Shishupal etc. fail to lift the bow. Only Karna can do it but he is denied to do so by Draupadi saying that he does not want to marry him; and Karna accepts the thoughts of Draupadi and sits down.
See it fabulously presented in Ramanand Sagar's Krishna serial