This study analyzed adverse effects of fusion plants on the stability of small-scale grids through simulation-based case studies. Major frequency fluctuations were observed in the event of a plant start-up and a sudden interruption of fusion electric output power, as represented by plasma disruption, suggesting difficulties in fusion plant installation to majority of countries in the future. To mitigate the adverse effects, a fusion plant combined with an energy storage was proposed and evaluated. The results indicated that large energy storage like pumped-hydro storage could successfully reduce the frequency deviations to the acceptable range. An innovative offshore pumped-hydro storage, marine inverse dam, is also discussed in this paper.