The murky death of Joe Walsh, General Secretary of the Crane Driver's Union, whose body was found in the Thames in 1975, is reinvestigated. At the time of his death, Walsh stood accused of stealing money from the union's account. The team find themselves at the centre of a political minefield, with suggestions of misappropriated union funds, MI5 involvement and a union leader whose threats of strike action were gravely threatening the national interest.