There have been a lot of audio anniversaries recently - 50 years of Doctor Who in 2013, 15 years of Doctor Who at Big Finish in 2014, and in 2015 the Doctor Who monthly range clocks up its 200th release!

'I find it almost incredible that we've got this far,’ says executive producer Nicholas Briggs. ‘The Doctor Who main range has been our backbone for so long.'

And so, in April, May and June, for releases 198,199 and 200, the company is doing something very special indeed, which is all the masterplan of script editor Alan Barnes.

‘For some time, I'd been thinking it was a shame we couldn't make more use of the first three Doctors' companions in the Main Range,” says Alan. ‘Continuity permitting (or even not!), we can always have the Fifth, Sixth or Seventh Doctors meeting up with friends they said 'goodbye' to years before... but there have been a fair few School Reunion-type stories now, and they're great, but I couldn't help wondering: can't we do something different?

‘Then I thought: what if the Fifth, Sixth and Seventh Doctors turned up bang in the middle of earlier, unseen adventures – standing in for their 'proper' selves, who've gone AWOL for some reason or other...? Locum Doctors, if you will! ‘And that's what we've done!’

First up is The Defectors by Nicholas Briggs, which pairs Sylvester McCoy with the Third Doctor’s companion Jo Grant, while Richard Franklin also appears as Captain Mike Yates. The guest stars in this first story of the trilogy are David Graham (Thunderbirds, Doctor Who - Dalek and Cybermen voices, and Kerensky in City of Death) and Barnaby Edwards (Dalek).

‘The challenge is to put your displaced Doctor into an adventure which feels like it comes from another era, and he has to work out why he’s here and what he’s supposed to do,’ says Nick Briggs. ‘It brings up lots of questions about what makes the Doctors different. This one is a different spin on the “base under siege” scenario, set on a remote island, but still involving lots of government and UNIT shenanigans.'

May’s release is Last of the Cybermen, which is written by Alan, and reunites Colin Baker with Frazer Hines and Wendy Padbury. ‘It finds the Sixth Doctor joining forces with Jamie and Zoe to explore a strange citadel that holds the secret of the Cybermen's ultimate destruction,’ says Alan.

Finally, release 200 itself is Eddie Robson's The Secret History – in which the Fifth Doctor, trapped with Steven and Vicki in sixth century Constantinople, at last learns the reason why he's being dragged back into the lives of his former incarnations. Peter Davison, Peter Purves and Maureen O’Brien are joined by guest stars Lysette Anthony (Dark Shadows, Oliver Twist) and Tony Millan (Citizen Smith).

But if any of those Doctors changed their own past, even in the smallest way... what might happen to their future?’ Alan teases.

Doctor Who - The Defectors, Doctor Who - Last of the Cybermen, and Doctor Who - The Secret History are all available to pre-order now, or can be purchased as part of either a six- or twelve-story subscription at a special price along with unique extras...