Jeremy Gill is on the dock at Gill Point on the northern shore of Flat Iron Lake. He will give you the Stranger mission "A Fisher of Fish" and the Legendary Fish Map.

You don't need the mission or the map to catch Legendary Fish but it will make it significantly easier. You need to have the Legendary Fish Map to be able to buy the Legendary Fish Special Lures from the bait shop. The special lures make it much easier to catch the Legendary Fish.