A trip for the Doctor and Heather to Hyde Park in the future finds it paved over and being used as a car park. Racing to a sudden explosion at the Royal Albert Hall, they find panic and chaos. From the explosion and derelicts on the streets they see a band of rebels trying to escape from an army of hissing, highly mobile tripods, each housing a plant in their domed structures, chanting in unison "Monoxise the humans. Monoxise!"