Fantastic was recruited by the NCR because they required technical experts to work on the HELIOS One power plant. When asked if he'd be qualified, he claimed to know as much about solar power as anyone he knew, and also boasted a "theoretical degree" in physics. This was enough to satisfy the NCR recruiters, and he was given the job. If told that he doesn't know what he's doing, he disagrees and insists that he knows exactly what he is doing, he just doesn't know what effect it will have. When asked to elaborate on his working environment, his explanation starts promisingly enough - "That console there controls the main power grid." - but soon degenerates, ending with "That slot over there makes sparks when you put stuff in it."
If chosen to wipe out the NCR in That Lucky Old Sun by arming Archimedes I, and Fantastic is left alive, upon returning it will be seen that Fantastic will be wearing Legion armor and will have killed Ignacio Rivas. If they then talk to him he will say "Hey man, when in Rome."
Alternatively, if chosen to side with the NCR, Fantastic can be found at Hoover Dam. He gets a promotion to the position of "God Damn Dam God" in his own words.
When first meeting Fantastic, it can be stated they're there to replace him. Fantastic gets worried and tells them that he needs this job for his chem addiction, and with a Speech check of 25, 100 caps can be extorted from him in return for not taking his job.
If chosen to send power to McCarren and The Strip in That Lucky Old Sun, an NCR guard will remark "Maybe that guy Fantastic wasn't an idiot, maybe he was a genius," when exiting the facility.
"No, man. I know exactly what I'm doing. I just don't know what effect it's going to have. Over there controls power in this building. That station has readouts on the computer network. That big knob there makes a crazy noise. Sparks come out of that slot if you put stuff in it. And I'm learning more every day."