You figure out how to put a fusion reactor on a spaceship without melting the whole thing into a blob of metal and use a particle accelerator as a super high isp engine.

Sci-fi but mostly in the ’not melting down’ part.

Could you elaborate this? Is the melting because you suddenly have lot of energy and no way for cooling?

Yeah you need to work very hard to lose heat without losing mass in space. Basically you can only radiate it away, which is slow; there are ways of concentrating heat in one spot so it is radiated more efficiently, but that’s where you get to the melting part.