Originally transmitted in 1961, The Avengers was a groundbreaking crime adventure series, following the activities of John Steed, a suave secret agent investigating Britain's strangest crimes - aided by the dashing Dr. David Keel. Together, they pitted themselves against the underworld's most devious and colourful villains.

Sadly, 24 of the 26 episodes of the first series no longer exist; but Big Finish have revisited these missing episodes in a series of audio box sets, starring Anthony Howell as Dr. Keel, Julian Wadham as Steed, and Lucy Briggs-Owen as Carol Wilson.

Now available to buy on CD and download, The Avengers – The Lost Episodes: Volume 6 brings to life three more classic episodes, adapted by John Dorney, Rae Leaver and Ian Potter. Listen to the trailer below:

6.1 The Frighteners by Berkeley Mather, adapted by Rae Leaver

If you need someone scared off, you speak to the Deacon. Steed and Keel are on the trail of an intimidation expert whose services have recently been acquired by a wealthy businessman. But as they head deeper and deeper into this seedy world, who do they really have to stop. And who do they have to save?

6.2 Death on the Slipway by Ian Potter, from a storyline by James Mitchell

When a fellow agent is killed, Steed is sent undercover at a government dockyard to find the killer. Can he sort the truth from the lies and track down the enemy infiltrator hunting for top secret plans before it’s all too late?

6.3 Tunnel of Fear by John Dorney, from a storyline by Terrence Feeley

Southend. A perfect spot for a family holiday - or for a traitor to hide. Somewhere in the town lurks a spy ring that is smuggling out classified information. When his old mole is attacked, and with innocent lives at risk, Steed takes Keel to the seaside for a far from sunny time.

Guest-starring Hugh Ross, Jon Culshaw, Barnaby Edwards and Dan Starkey, The Avengers: The Lost Episodes Volume 6 is available to buy and download today from the Big Finish website. Listen immediately with the complimentary digital download included with CD orders. With the set not available on general release until August 31st 2016, it's the perfect chance to revisit the classic episodes before anyone else!

Five previous volumes of Lost Episodes can also be ordered today, with the final collection available to pre-order ahead of its release on the Big Finish website in January 2017.

The Avengers return in November with The Avengers – Steed and Mrs Peel: The Comic Strip Adaptations Volume 2, starring Julian Wadham and Olivia Poulet. The much-praised Volume 1 is available now and both box-sets - along with a companion graphic novel - can be purchased together as part of a money-saving bundle.

Fans of surreal, ground-breaking 1960s cult TV should check out Big Finish's acclaimed The Prisoner  range too...