In search of the key to a sinister alien technology, the Doctor, Klein and Will set their sights on an illicit intergalactic arms fair run by an old acquaintance - the slippery Urodelian crook, Garundel. But what are their chances of pulling off a particularly audacious heist from under the noses of Garundel and his alien clients, the ever-belligerent Sontarans.

The universe stands on the brink of a dimensional crisis - and the Doctor and Raine are pulled into the very epicentre of it. Meanwhile, on Earth, UNIT scientific advisor Dr. Elizabeth Klein and an incarnation of the Doctor she's never encountered before are tested to the limit by a series of bizarre, alien invasions.