For Tube drivers, their worst fear is somebody jumping or falling in front of their train. We follow what happens when this fear becomes a distressing reality for two drivers. And we meet the crews of the Tube's specialised emergency response units as they're scrambled to deal with any eventuality on the Underground, from accidents and injuries to mysterious obstructions on the line. Plus how the Tube and its dedicated British Transport Police officers cope with the demands of the Notting Hill Carnival.