Every summer weekend, London's Leicester Square becomes the front line in the war against litter. Tonight street sweeper Donovan reveals how he suffers verbal abuse and even violence from drunk clubbers as he attempts to clear their ankle deep litter. And the team has to call in an expert, John the Flusher, to spray disinfectant in an alley regularly used as a public toilet. In an attempt to find out why we drop litter in some places, but not in others, Margaret Mountford stages a fascinating psychological experiment. The target is cyclists, who will have to dispose of leaflets the team has attached to their bikes. But will more of them drop leaflets in the area that has been deliberately littered, proving that people become litter louts if they think everyone else is doing the same? And the uplifting story of canoeists reversing the tide of litter on Leicester's River Soar. The waterborne volunteer litter pickers gather 30 bags of rubbish from the water every month.