Thrym, King of the Jotunn (Giants), steals Thor's mighty hammer Mjölnir. Thrym then demands that Freyja, goddess of sex and beauty, marry him. The gods decide to accept Thrym's demands, in a way.
As part of the ceremony, Mjölnir is placed in the "bride's" hands. Thor reveals his deception and uses the hammer to overthrow the giants.
To read the entire story in the Eddas, click here.