Mission Improbable was the thirty-fourth Subscriber Short Trips audio story. It featured the Sixth Doctor and Evelyn Smythe and was available to subscribers of Big Finish's main Doctor Who range whose subscription included Serpent in the Silver Mask.
In a prison thirty miles from the mainland, Evelyn is visited by Captain Gaines, who claims that there is footage of her and the Doctor taking part in a rebellion. She is sure that the Doctor will rescue her in the TARDIS before the time of her execution, but he instead infiltrates the prison by jumping from his flying ship with a parachute and putting on some overalls in a disguise that he completes with a false moustache.
The Doctor, after evading two security guards and causing a third to knock himself out, locates Evelyn by accessing the CCTV at a computer terminal and attempts to shut down the security systems on the route. His biometrics are not recognised, however, and he sets off an alarm by damaging the computers with a wet mop before travelling through the air vents. Later, Gaines tells Evelyn that the Doctor has been captured and that he too will be executed in the morning, but, when he exits, she calmly collects her handbag and prepares to leave.
Disguised as a guard, the Doctor closes the door behind Gaines and frees Evelyn, congratulating her on her acting abilities and explaining that he took his guard's helmet from a new member of staff who was subsequently misstaken for him. They leave, intending on letting the wrongfully-imprisoned guard out, and the Doctor rhetorically asks Evelyn who could ever forget her.