Fergus mac Leti, the king of Ulster who was an accomplished swimmer was kidnapped by water-spirits. They brought him down into the water but he was awoken by the cold, this allowed him to capture the spirits and he demanded from them 3 wishes. He wished to be able to breath under water in seas, pools and lakes, and the wishes were granted with one condition. He was not to use his gifts at Loch Rudraige in his own land. In this Loch he encounters Muirdris, an undefinable horror.

Fergus is horribly disfigured from the encounter. His courtiers somehow kept the disfigurement a secret from the king for 7 whole years before a slave taunts his about his appearance. Knowing this to have been caused from the encounter the king goes to face Muirdris. The battle lasts a day and a night during which the Loch began to bubble like a cauldron. Fergus finally defeats Muirdris with his hands and in his triumph collapses to the ground in death himself.