Its surface ravaged by colliding time-fields, the planet Hirath is a patchwork of habitable areas separated by impenetrable zones of wild temporal fluctuations.

The planet's unique biosphere is being exploited by an uncaring company happy to rent out temporarily isolated chunks of the planet to the highest bidder - no questions asked.  But the controlling computer seems to be malfunctioning, and the viability of the whole planet hangs in the balance - along with countless thousands of lives.

Arriving at Hirath's control base, the Doctor and Sam are soon separated and trapped on the dying planet.  While Sam becomes the focus of attention in a barren penal settlement, the Doctor discovers the secret of Hirath's unique condition - just as a race of hideous bloodthirsty alien creatures arrive in force to reclaim it.

Caught up in a desperate struggle for survival, it seems time has run out for every living creature on Hirath - not least Sam and the Doctor...