Bessie was a canary-yellow roadster, acquired by the Brigadier for the Doctor's use as part of the exiled Time Lord's agreement to assist UNIT in its efforts to protect Earth. The Doctor had requested a car of this nature after using a similar vehicle in his first fight against the Autons and liking its style and character.
The Doctor made several modifications to Bessie during his time at UNIT. These included an anti-theft force field, which magnetically trapped anyone attempting to steal Bessie; a remote control, with which the Doctor could steer the car from a distance; and a minimum inertia hyperdrive which allowed him to drive at breakneck speed and still come to a sudden stop without him or his passengers being sent through the windscreen by inertia, which the brakes absorbed.
When he had begun his exile on Earth, the Doctor (who had just regenerated and not fully recovered) stole an antique roadster parked in front of Ashbridge Cottage Hospital so that he could get around. He liked it, and, while seeing to it that the car returned to its rightful owner, the Brigadier said he thought he could get something like it.
Bessie proved instrumental in capturing the Master. When he attempted to steal the Doctor's car and make good his escape following the events at Devil's End, the Doctor used his remote control to steer the Master back to the waiting UNIT soldiers, who took him into custody. Bessie was briefly pulled into Omega's universe during his attempt to capture the Doctor.
Shortly after his companion Jo Grant left him the third incarnation of the Doctor had temporarily left Earth, his fifth incarnation arrived on Earth and borrowed Bessie while helping UNIT defeat the Xaranthi.
The car was also abducted along with the Doctor into the Death Zone, where it was heavily damaged by one of the thunderbolts that periodically rained down upon the place.
With his waning lack of interest in affairs in UNIT and Earth, the newly-regenerated Doctor only drove Bessie during his first adventure. At some point after this, Brigadier Lethbridge-Stewart had it "put into mothballs".
It was brought out briefly in the 1990s as the Brigadier and UNIT worked together with the Doctor's seventh incarnation to battle Morgaine.