August 1968. Czechoslovakia is enjoying a period of political liberalisation. Alexander Dubček is trying to secure his country greater liberties, but it will come at a cost. When the TARDIS brings the Doctor and Kim-Ly to Prague, this freedom is nearly at an end, and the travellers are able to see life on the cusp of the Soviet invasion.
Married couple Ludvik and Jozka want to play music and raise their daughter, but the threat of conflict is forcing them to decide how far they will go to keep their family safe. Hapless diplomat Alistair wants to make a difference, but his boss, Valentine, refuses to intervene, and he’s worried that the woman he loves is getting involved in dangerous matters.
But it isn’t only the advance of the Red Army that they have to worry about: the alien Koe-Maw are also visiting Prague, and the Doctor wonders whether their promise of salvation is all it seems. As a dark day approaches, can the Doctor and Kim-Ly keep the Czech people from making a cataclysmic mistake that would change the fate of not only the Prague Spring, but of the entire world?