Upon arriving in Klamath, head over to the Buckner House and talk to Ardin. When you ask what's bothering her, she tells you her fiancé, Smiley has gone missing, his last known location being the Toxic Caves. Agree to keep an eye out, and she'll mark the location on your map.
You'll need some rubber boots to get by without getting radiated and there are many geckos running about that you can either fight or avoid. Smiley is located on the 2nd floor, hiding in a generator room. Talk to him and bring him to the entrance where he'll thank you and leave your party. Head back to Klamath for your reward.