The TARDIS materialises not far from Paris in 1794 - one of the bloodiest years following the French Revolution of 1789. The travellers become involved with an escape chain rescuing prisoners from the guillotine and get caught up in the machinations of an English undercover spy, James Stirling - alias Lemaitre, governor of the Conciergerie Prison.

The Doctor - posing as a Regional Officer of the Provinces - is twice brought before the great tyrant, Robespierre himself, and has to talk himself out of trouble. Ian and Barbara, meanwhile, have a close encounter with a future ruler of France, Napoleon Bonaparte.

As events reach their climax, Robespierre is overthrown - shot in the jaw and dragged off to the prison - and the Doctor and his friends slip quietly away.