Bessie

Though other incarnations of the Doctor used the car, Bessie was the Third Doctor's main mode of transport during his exile on Earth. A canary-yellow Edwardian roadster, she was a key part of his compensation package from UNIT, and indeed the only specific demand he made of the Brigadier when he agreed to be UNIT's scientific advisor. (TV: Spearhead from Space) Despite being beloved by the Doctor, the car was frequently ridiculed and underestimated by the Doctor's friends. Heavily tricked out, the car's many modifications often bewildered his associates. (TV: The Dæmons, The Time Monster, Battlefield) The Brigadier in particular was dismissive of the car's abilities and, at least early on, didn't believe it was capable of basic locomotion. (TV: The Ambassadors of Death)

Modifications
The Doctor made several modifications to Bessie during his time at UNIT. These included an anti-theft force field (TV: The Ambassadors of Death), which magnetically trapped anyone attempting to steal Bessie; a remote control (TV: The Dæmons), 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 the car's occupants being tossed through the windscreen by inertia, which the brakes absorbed. The Doctor made use of this inertia absorption to overtake the Brigadier's Land Rover after Lethbridge-Stewart had declined to travel in Bessie in fear of the car's reliability. (TV: The Time Monster)

History
When he began his exile on Earth, the Third Doctor (who had just regenerated and not fully recovered) stole an antique roadster parked in front of Ashbridge Cottage Hospital to get around. He liked it and while seeing the car returned to its rightful owner, Doctor Beavis, the Brigadier said he thought he could get something like it. The original car was red, unlike Bessie, which was yellow. (TV: Spearhead from Space)

Bessie proved instrumental in capturing. 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. (TV: The Dæmons) Bessie was briefly pulled into Omega's universe during his attempt to capture the Doctor. (TV: The Three Doctors) Shortly after his companion Jo Grant left him and the third incarnation of the Doctor had temporarily left Earth, his fifth incarnation arrived on Earth and borrowed Bessie while helping UNIT defeat the Xaranti. (PROSE: Deep Blue)

Bessie was also abducted with the Doctor into the Death Zone, where it was heavily damaged by one of the thunderbolts that periodically rained down upon the landscape. (TV: The Five Doctors)

With his waning interest in affairs at UNIT and Earth, the Fourth Doctor soon lost interest in Bessie, though he did have use for her on several Earthbound adventures with Sarah (TV: Robot, COMIC: Death Flower, Counter-Rotation, Mind Snatch, and others). He also used Bessie after Sarah was no longer travelling with him, (COMIC: The Fire Feeders) and even when Leela had joined him (COMIC: The Devil's Mouth)

It was unclear what happened to Bessie after this early period of the Fourth Doctor's life. According to one account, it was simply in storage aboard the TARDIS, seen there as late as the Sixth Doctor's travels with Frobisher and Peri. (COMIC: Changes) Another account had it that the Brig had it "put into mothballs" at some point, having realised that the Doctor's visits to Earth were becoming more and more infrequent. It was brought out briefly in 1997 as the Brigadier and UNIT worked with the Doctor's seventh incarnation to battle Morgaine; it was briefly criticised by Ace, until the Doctor demonstrated its capabilities. (TV: Battlefield).

The Ninth Doctor used Bessie at one point, and had been seen trying to get it running after it had broken down at the side of the road. (WEB: whoisdoctorwho.com)

Technology
Bessie had a built in Gravity stabiliser. (COMIC: The Vortex)

Registration
Initially, Bessie carried the registration license plate WHO 1. Whether this was intentional - possibly chosen by the Doctor as a reference to the question often asked of his name - or a coincidence is not known. By seeming coincidence, at the time of the Seventh Doctor's encounter with Morgaine, the plate had been changed to WHO 7. (TV: Battlefield) When the Eighth Doctor utilised Bessie during the 1997 invasion of Britain, a UNIT soldier reported the number plate as "Whiskey, Hotel, Oscar 8," that is, WHO 8. (PROSE: The Dying Days)

Behind the scenes

 * The actual registration WHO 1 was already in use in real life by another motorist, so Bessie was actually registered under a different number plate, MTR 5. Special WHO 1 number plates were made, which were used on the car when filming on private ground or on the public highway with permission from the police.
 * Bessie may possibly have been named in honour of William Hartnell's aunt, Bessie Wood, who had looked after him during his troubled childhood.
 * A photograph of Jon Pertwee driving Bessie appears in the frame of a mirror owned by Pertwee's character, Paul Henderson, in the 1971 film The House That Dripped Blood. At the time of filming, Pertwee was still playing the Doctor.
 * Phil Collinson said he paid 50p for a ride on Bessie round a shopping centre when he was a child.
 * Model car manufacturer Corgi made a Bessie in a series of models built to commemorate an anniversary of Doctor Who.