UNIT: an international initiative to investigate and confront alien attacks across the planet. During more than 50 years of service, UNIT have defended the Earth against invading Autons, Cybermen, Sontarans and more – holding the line, thanks to the heroic and often covert actions of a small team of brave individuals, including their mysterious Scientific Advisor, the Doctor.

Today, Britain is protected by Kate Stewart (Jemma Redgrave), the daughter of the legendary Brigadier Lethbridge-Stewart (Nicholas Courtney) – and the leader of a team who fought bravely against the Nestene Consciousness in our recent box-set release, UNIT: Extinction.

But 20 years earlier, the UK team was led by the equally dynamic Dr Elizabeth Klein (Tracey Childs) – a scientific genius who travelled with the Doctor in a previous life. Now she stands at the frontline of an inter-dimensional catastrophe in UNIT: Dominion – available this weekend for just £20 on CD or £15 to download!

This epic five-disc boxset penned by Nicholas Briggs and Jason Arnopp sees Klein's UNIT team join forces with Ace (Sophie Aldred), Raine Creevy (Beth Chalmers) and two incarnations of the Doctor (Sylvester McCoy and Alex Macqueen).

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.

At the heart of it all is a terrible secret, almost as old as the Time Lords themselves. Reality is beginning to unravel and two Doctors, Klein, Raine and all of UNIT must use all their strength and guile to prevent the whole of creation being torn apart.

Pick up UNIT: Dominion today for just £20 on CD or £15 to download. And remember, when you choose to buy this release on CD directly from Big Finish, you will also unlock instant access to a digital copy for download.

Order today buy hurry; this offer must end noon Monday 5th December (UK time)

Further UNIT operations can be enjoyed in a variety of releases from across the worlds of Doctor Who.

Brigadier Lethbridge-Stewart leaps to an old friend’s defence in our Main Range stories Doctor Who: The Spectre of Lanyon Moor and Doctor Who: Minuet in Hell, and joins an alternative Doctor (David Warner) for two adventures: Doctor Who – Unbound: Sympathy for the Devil and Doctor Who – Unbound: Masters of War.

In The Companion Chronicles, Scientific Advisor Dr Liz Shaw (Caroline John) strikes out on her own to investigate terrifying new threats in Doctor Who: The Blue Tooth, Doctor Who: Shadow of the Past, Doctor Who: The Sentinels of the New Dawn, Doctor Who: Binary and Doctor Who: The Last Post.

UNIT agent Jo Grant (Katy Manning) and Captain Mike Yates (Richard Frankin) join the Third Doctor in Doctor Who: The Third Doctor Adventures Volume 1, while Sergeant Benton (John Levine) is dispatched on a special mission in Doctor Who: Council of War. A decade or two later, Battlefield veteran Brigadier Bambera (Angela Bruce) aids the Seventh Doctor's struggle against the Metraxi in Doctor Who: Animal.

Then in the early 21st Century, the Brigadier (Nicholas Courtney) returns to action to help a new UNIT team face a Britain divided. UNIT, a four part mini-series, stars Siri O'Neal as Colonel Chaudhry, Nicholas Deal as Colonel Dalton and David Tennant as Colonel Brimmicombe-Wood. All four episodes are available at £5 each, or in a bundle for £15. The prequel episode, UNIT: The Coup, can be downloaded entirely free of charge from the Big Finish website.

And in a secret bunker under the Angel of the North, the specialist team of Captain Ruth Matheson (Daphne Ashbrook) and Warrant Officer Sato (Yee Jee Tso) keep a watch on confiscated alien artefacts. Doctor Who: Tales from the Vault reveals the truth behind four of the assets – while Doctor Who: Mastermind delves into the life of the Vault's most secure prisoner: the Master (Geoffrey Beevers).

Finally, for a more personal insight into the UNIT Years, Nicholas Courtney’s memoirs, A Soldier in Time, recalls his time on Doctor Who, from The Web of Fear to Battlefield, and beyond.

In May 2016, UNIT's new recruits will take on their next alien threat in UNIT: Shutdown – with the episode titles now revealed on the product page! The set is available to pre-order today for just £20 on CD or download – with a special bundle of all four UNIT: The New Series box-sets also available to order with an additional discount. Check out the video trailer for the first in the series below: