Doctor Who: Sontarans vs Rutans - Born to Die

In Born To Die, Colin Baker features as the Doctor, accompanied by India Fisher as Charley Pollard. This full-cast audio adventure is set during the period when Charley found herself in the far future, separated from the Eighth Doctor, before being rescued by the Sixth Doctor.
When a trip to the planet Taxodon to see its native species, the Knox, goes wrong the Doctor finds himself with mere hours to prove himself innocent of murdering a Sontaran. With more Sontarans dying, the Doctor and Charley need to work but what is killing them and why the Sontarans are even on Taxodon in the first place… if they have time…