Past Lives (audio story)

 was the first story in the Once and Future audio series, produced by Big Finish Productions. It was written by Robert Valentine and featured Tom Baker as the Fourth Doctor, Sadie Miller as Sarah Jane Smith, Jemma Redgrave as Kate Stewart, Ingrid Oliver as Petronella Osgood and Rufus Hound as.

Publisher's summary
The Time War. The Doctor has been injured and brought to a Time Lord field hospital. His body glows with energy, but this is no regeneration into a future form – instead, the Doctor's past faces begin to appear as he flits haphazardly between incarnations...

Staggering to his TARDIS, the Doctor sets out to solve the mystery of his 'degeneration'. Who has done this to him? How? And why? From the Earth to the stars, across an array of familiar times and places, he follows clues to retrace his steps, encountering old friends and enemies along the way. Tumbling through his lives, the Doctor must stop his degeneration before he loses himself completely...

Settling as his Fourth incarnation, the Doctor goes in search of the Monk, with a vague memory that he had something to do with his 'degeneration'.

On Earth, the Monk is meddling, bringing Sarah Jane Smith to the future UNIT HQ to steal a device for an alien race. The Doctor must help Kate Stewart and Osgood foil an invasion before he can confront the Monk about what he knows…

Plot
During the Time War, the Doctor biogenically reverts through their incarnations and runs to the TARDIS, finally stabilising in their fourth incarnation in part due to the ship's calming influence. He uses a temporal iterator to trace the Monk to an island off the coast of Scotland in the 20th century, remembering that he has some involvement with what is happening, and follows him to 22nd century Hong Kong where the Monk exchanges diamonds for a smart black omnivariable decryptor before sedating him and running off. After meeting with Mr Mallory at Glastonbury Tor, the Monk kidnaps Sarah from Aberdeen.

The Doctor follows the Monk to London in early 2010, but detects a cloaking device above Glastonbury which Kate and Osgood are also aware of and goes to investigate, meeting the two of them in the Tower of London just before a Hyreth chasing them dies of old age. They head down to the Black Archive to collect the Hyreth's energy source, the Magna Gral, and discover that the Monk has used the decryptor to break in with Sarah, who has fought the Hyreth in the past, to find the device. The Monk explains that he has had nanobombs put into his blood by a client before taking the Magna Gral to his TARDIS, frying the Doctor's temporal iterator before dematerialising.

Osgood reveals that she has been completing an iterator that the Third Doctor worked on during his exile and they use it to trace the Monk to Glastonbury where Mr Mallory, who is actually the Hyreth Galavaunt, takes the Monk and the Magna Gral to his warship to awaken his people. The Doctor materialises aboard and orders Kate and Osgood to guard the TARDIS whilst he and Sarah go to speak with the Monk, but they ignore him and confront Galavaunt as he attempts to use the Magna Gral, which succeeds only in giving the Hyreth enough life to reach their natural end and die. Galavaunt mourns his people and surrenders himself and Baravoar to UNIT, who intend to place him in a care home for aliens.

The Doctor neutralises the Monk's nanobombs and learns that the Time War is in his personal future, meaning he has no knowledge of the degeneration weapon and can only suggest that he visit his daughter to find a way to delay the weapon's effects with sympathetic DNA. The Doctor tasks the Monk with returning Sarah to her own time and begins to change his appearance once again, saying goodbye to Sarah, Kate and Osgood and leaving in his TARDIS, promising to continue to remember that UNIT is there for him.

Cast

 * The Fourth Doctor - Tom Baker
 * Sarah Jane Smith - Sadie Miller
 * Kate Stewart - Jemma Redgrave
 * Osgood - Ingrid Oliver
 * The Monk - Rufus Hound
 * "Mr Mallory" / Galavaunt / Baravoar - Ewan Bailey
 * Field Surgeon / Merchant - Dan Starkey
 * The First Doctor - Stephen Noonan
 * The Second Doctor - Michael Troughton
 * The Third Doctor - Tim Treloar
 * The Fifth Doctor - Peter Davison
 * The Sixth Doctor - Colin Baker
 * The Seventh Doctor - Sylvester McCoy

Continuity

 * Sarah had just left the Fourth Doctor moments earlier when she confronts the Monk. (TV: The Hand of Fear)
 * The Monk has put the money he had stolen into a Swiss bank account. (TV: The Time Meddler)
 * Sarah has met the Monk before. (AUDIO: How to Win Planets and Influence People)
 * The Doctor admires the celery on Osgood's jacket. (TV: Castrovalva, et al)
 * The Monk is aware that the Doctor will have a daughter, much to the Doctor and Sarah's surprise. (TV: The Doctor's Daughter)