It's 1794, and the TARDIS materialises some distance from Paris during the French Revolution, the scene of the infamous Reign of Terror. Soon the Doctor, Susan, Ian and Barbara find themselves caught up in the tangled web of historical events. Who is James Stirling, the master-spy to whom Ian must deliver a message? What world-shattering events are being discussed in a deserted inn off the Calais road? And can the Doctor and his friends escape a violent and bloody death at the dreaded guillotine?