Invasive species are a danger to the ecosystem, threaten such areas as forests, waterways and even cities. `Infestation' examines some such species, explaining why they are a concern, and taking a look at current and past infestations in addition to potential threats in the future. Episodes highlight the roles of the scientists who work to tackle the challenges presented by each species, focusing on the effects at both the local and global level - including globalisation trends, global warming and the spread of disease. The stories of people who have been personally affected, often paying a hefty price, are also told.