What strange attraction lures people to the planet Drebnar? When the TARDIS is dragged there, the Doctor determines to find out why.
He discovers that scientists from the mysterious Frontier Worlds Corporation have set up a base on the planet, and are trying to blur the distinction between people and plants. The TARDIS crew plan to prevent a biological catastrophe - but their plan goes wrong all too soon.
Compassion finds her undercover work so engrossing she risks losing her detachment. Fitz seems too distracted by the local population to keep his eye on Compassion. So when the Doctor gets trapped in a freezing wilderness, who can stop him falling victim to a lethal experiment in genetic modification?
For something else has been lured to Drebnar, something that Frontier Worlds Corporation will ruthlessly exploit without care for the consequences - an ancient organism which threatens to snuff out Drebnar's solar system.
Outside the ordered universe, things move. They're hungry. And something has given them the scent of our space/time.
In the far future, the Doctor has learnt of the war and feels he must intervene - but it's more than just a local conflict of interest. One of the groups of combatants is from his own future, and the other has never, ever, existed.