The main issue I see is that one really can't "just have fusion power" in the 70s, any more than we can just have machine learning. Fusion is a fiendishly difficult proposition, well beyond the technology of the time, so I think you'd have to have something SFional that acts like fusion, yet really isn't.
So here's a few possibilities:
- Runaway: Unlike normal fusion, this is a zero-point energy process that extracts energy from the Quantum Foam using a self-sustaining field effect. Using it to heat is how you get a working fluid to run a magnetohydrodynamic power plant. The problem is, if it runs wild, they're may be no stopping it.
- Product of a crashed UFO: Flying Saucers are very 70s. It's basically a weird literally black box technology that we really don't understand, though we can duplicate them. Actually, we don't-they duplicate themselves under the exactly right conditions. And if we make too many of them, the aliens will notice. Better set up a UFO defence agency hidden as a movie studio...
- Espers: what's more 70s than ESP? This fusion system requires telekinetics to, I dunno, convince the plasma to choose it something. But aside from the limited supply of espers, there's the fact that some of them might not want jobs as living power generators, or others may have more selfish, sinister uses for their abilities...
- Dimensional Weirdness: you get your energy by opening a crack in the space tone continuum. This can have all kinds of weirdness ranging from time and space distortion, time ghosts, people from alternate universes, dinosaurs...and the longer it goes on, the more "corrupted" the area gets (The Zone, The Southern Expanse, etc.)
- It's an evil plot: But they were, all of them, deceived, for another Fusion Reactor was made. In the land of California, in the fires of Stanford, the Dark Lord Teller forged in secret a master Fusion Reactor, to control all others. And into this Fusion Reactor he poured his cruelty, his malice and his will to dominate all life. One Fusion Reactor to rule them all...