With the Houses of Parliament on the banks of the Thames, the river provides a perfect vantage point for planning crime and terror. So the Marine Support Unit polices a 50-mile stretch of tidal river 24 hours a day. They don’t just deal with potential terrorism, they also field suicide attempts and the occasional, ill-advised swimmer. They’re “the only unit in London specially trained to preserve evidence and retrieve bodies from water”.

This dramatic documentary will make you wonder why no one has made a British crime drama about river police … yet.