The Day of the Troll is an exclusive to audio Doctor Who story, produced as part of BBC Books' New Series Adventures line, and the fifth entry in the series to be produced. Written by author Simon Messingham and read by series star David Tennant, it features the Tenth Doctor travelling alone. It was released on CD on 8 October 2009.
Siblings Karl and Katy are in an old, scary forest – Karl winding his sister up. We learn that Katy is a former "medical type", and that the two are involved in experiments – experiments that are being ruined by something.
Charlie and the Doctor drive a 4x4 towards the bridge. The Doctor asks for details on the Grange. Charlie explains that humans used a process dubbed Global Cooling, refreezing glaciers. The ground left behind was poisonous, and food became incredibly value, leading to violence. Karl Baring started work on growing new crops, but so far, hasn't succeeded – running out of money, depending on charity.
Sanders contacts Petra to tell her the relay link's down – they can't contact Madrid. Either they were shut off in Madrid, or the local relays have been sabotaged. Suddenly, Campbell is grabbed by the troll. The Doctor manages to grab it, and recognises its face – it's Karl Baring. They struggle, the Doctor is knocked down, and the troll attacks.