Osgood and Captain Josh Carter are sent to investigate the disappearance of a UNIT scientist.

Meanwhile, alien technology has fallen into the hands of Lyme Industries, and Kate Stewart can't persuade the company’s CEO, Felicity Lyme, to give it back.

But UNIT find themselves fighting a third battle when innocent people start to die. Who are the mysterious assassins? And what does Felicity Lyme want with top secret alien technology?