[1] It is a rare example of inter-novel continuity in MacLean's writing, as one of the characters in his previous novel Puppet on a Chain makes a re-appearance.

A mysterious terrorist organization known as the "FFF" has detonated a mine which bursts dykes in the Netherlands and caused massive flooding of Amsterdam's Schiphol Airport.

Unless their demands are met (i.e. immediate withdrawal of all British military forces from Northern Ireland), they threaten to detonate more mines, flooding Holland beneath a wall of water from the North Sea.

Lieutenant van Effen is also an undercover operative with connections to a Dutch criminal gang; posing as a criminal explosives expert and with the help of fellow undercover officers Vasco (as a corrupt Dutch Army officer) and George (as a black market arms dealer), he sets out to infiltrate the FFF and sabotage their plans one way or another.

Matters, however, take an unexpected and dangerous turn when van Effen's sister Julie and Annemarie Meijer (an undercover policewoman and the daughter of a powerful Dutch industrialist) are kidnapped and held hostage by the FFF.