What do you get if you smash two hydrogen nuclei together? Helium and lots of energy. That’s no joke – it's nuclear fusion! Nuclear fusion is the power source of the sun and the stars. Physicists and engineers here on earth are trying to build reactors than can harness fusion power to provide limitless clean energy. But it’s tricky... Rutherford and Fry are joined by Dr Melanie Windridge, plasma physicist and CEO of Fusion Energy Insights, who explains why the fourth state of matter – plasma – helps get fusion going, and why a Russian doughnut was a key breakthrough on the path to fusion power. Dr Sharon Ann Holgate, author of Nuclear Fusion: The Race to Build a Mini Sun on Earth, helps our sleuths distinguish the more familiar nuclear fission (famous for powerful bombs) from the cleaner and much less radioactive nuclear fusion. And plasma physicist (another one!) Dr Arthur Turrell describes the astonishing amount of investment and innovation going on to try and get fusion power working at a commercial scale. Contributors: Dr Melanie Windridge, Dr Sharon Ann Holgate, Dr Arthur Turrell Producer: Ilan Goodman