Collection of four British crime thrillers from the 1950s. In 'High Treason' (1951), after the destruction of the SS Asia Star in London Docks, Commander 'Robbie' Brennan (Liam Redmond) joins forces with Special Branch and MI5 to investigate an underground terrorist group plotting acts of sabotage. They discover that the group are planning an attack on a power station. Can they stop them before it's too late? In 'The Big Chance' (1957), fed up with living his mundane life, travel agency employee Bill Anderson (William Russell) siezes his opportunity for a change when a customer returns tickets to Panama. Bill decides to take the tickets and go to Panama himself. While at the airport, however, he is distracted by the alluring Diana Maxwell (Adrienne Corri). When the flight is delayed until the following day, Diana manages to get Bill involved in all manner of misadventures. Will he be glad of this change from the humdrum of his daily existence? In 'Dublin Nightmare' (1958), adapted from the novel by Robin Estridge, Steve Lawlor (Richard Leech) is reported dead following a car accident after he helped a Republican gang rob a Northern Irish security vehicle. The loot has gone missing and while the gang believe the car passenger Danny O'Callaghan (Pat O'Sullivan) has betrayed them, Lawlor's former girlfriend is convinced he is still alive. His photographer friend John Kevin (William Sylvester) investigates. In 'Deadly Nightshade' (1953) Robert Matthews (Emrys Jones) is arrested in Cornwall when he is mistaken for convict John Barlow, to whom he bears a striking resemblance. When Barlow (also Jones) hears of this, he makes his way to the man's cottage and takes his place. After surviving a local shipwreck Robert's fiancée Ann Farrington (Zena Marshall) is taken in by Barlow, who maintains his imposture but soon discovers that Matthews is not all he seems.

After the destruction of the SS Asia Star in London Docks, Commander 'Robbie' Brennan joins forces with Special Branch and MI5 to investigate an underground terrorist group planning acts of sabotage. They discover that the group's next act of destruction is 'the big one' - an attack on a power station.