Parasite

'Change, Benny. It's the most terrifying thing of all.'
'And that's what's happening to you, is it, Doctor?'
'It's what's happening to all of us.'

The TARDIS has arrived in the Elysium system, lost colony of distant Earth and site of the Artifact: a world turned inside out, a world of horrific secrets. 

For more than a century scientists have studied the ecosystem flourishing within the Artifact. Now the system is in collapse and even the humans trapped inside are changing into something new and strange. 

With the members of one expedition murdered, those of another fighting for their lives and a solar system on the brink of civil war, can the Doctor, Ace and Benny survive a journey to the heart of the Artifact in their search for the truth