A mutation is a change in the genetic material of a living thing. This means changes to the DNA or to the chromosomes which carry the DNA, and all changes can be passed in to the next generation. Mutation can happen for lots of reasons, for example damage from radiation, certain chemicals, or just at random.
You will meet one scientist who is responsible for regulating Genetically Modified Organisms in Ireland, one researching cancer and one trying to find out why some animals can heal and regrow their bodies after an injury better than we can. There is also a scientist working out which bacteria or virus has caused infections in hospital patients and another working out if someone has tuberculosis, and whether this disease has developed mutations making it immune to treatment.