The planet Meridan is one of a thousand habitable worlds, apparently unremarkable from its fellows. It possesses a temperate climate, and a dominant species edging from the hunter-gatherer stage to the beginnings of rudimentary society.
However, when the Doctor arrives on the planet, events rapidly escalate away from the "unremarkable" end of the scale. An experimental Earth ship is forced to send an emergency away team down to the surface. But how is it that the natives already know English?
Is the planet nothing more than a pawn in the games of the Spinward corporation? Which of the Doctor's old enemies has taken a special interest in its inhabitants - and more importantly, why? What part does the placid pool of Meridan play in the twisted river of time?
Gradually, the Doctor realizes that while still waters may run deep, there's no way of knowing what might be lurking in those depths. And the rapids and cascades of time's river are unforgiving, and lethal.