First Doctor. First Sontaran encounter. First available from Big Finish. Doctor Who - The Sontarans!

Today sees the release of the final story of this year's run of Doctor Who - The Early Adventures with Peter Purves, Jean Marsh and Dan Starkey starring in Doctor Who - The Sontarans by Simon Guerrier:

The TARDIS arrives on a moon-sized asteroid orbiting two gas giants. With an amazing view, it’s a chance for the Doctor, Steven and Sara to unwind after their recent adventures.

But they quickly find themselves in the midst of battle - on one side: a familiar group of space-suited soldiers - members of the Space Security Service. On the other: strange, squat aliens in body armour.

Surviving the initial hostilities, the Doctor and his friends discover that the SSS squad is on a terrifying mission. With many lives at stake, they have to venture deep inside the asteroid in search of a hideous weapon. But who can they trust in the battle against these Sontarans?

Doctor Who - The Early Adventures: The Sontarans is available on Download for £12.99, or £14.99 for a CD release which - exclusive to Big Finish customers - unlocks a downloadable version for instant access. Pair this with the free Big Finish app to listen on-the-go if you have an iOS or Android device.

The first three series of Doctor Who - The Early Adventures can each be bought in a special-priced bundle collecting all four releases together for £40 on Download or £45 on CD. Next year's fourth series starts in September, and both this and the fifth series can be Subscribed to today.