The Doctor may appear human but he is in fact a Time Lord from the planet Gallifrey. He has two hearts, some limited psychic abilities and often displays unusual strength and speed (such as chopping a brick in half or typing faster than humanly possible). Most importantly, the Doctor can regenerate from a serious injury and come back with a new body and a slightly different personality. This allows different actors to play the same character. Different regenerations are referred to by their number (1st, 2nd, 3rd etc) or by the actor names (Jon Pertwee, Tom Baker etc).

The 4th Doctor is played by the actor Tom Baker. In contrast to the serious 3rd Doctor, the 4th Doctor has a very erratic personality and can be childish and prone both to sulking and to being excessively friendly to people who threaten him with guns (which happens a lot). He can be very sympathetic to those in distress and is particularly fond of the human race.

Despite the way the Doctor presents himself to the people he meets, he is indisputably an intelligent and experienced traveller and hero. He has very sophisticated scientific training and rarely encounters any technology that is beyond his understanding. Having travelled extensively, he is familiar with many forms of technology and types of life forms. He also carries some technology of his own.

The Sonic Screwdriver is a small device that emits a high pitched sound and can be used to explode mines, kill robots, burn through locks, destroy control panels and even unscrew bolts. The ultimate multi-purpose device.

The TARDIS is the Doctor's ship. The TARDIS (or Time And Dimensions In Space) can travel backwards and forwards through time as well as to nearly any point in the universe. The TARDIS is suppossed to morph into an inconspicious object before it materialises (becomes solid) but during a previous visit to Earth, the TARDIS malfunctioned and has remained in the shape of a blue police telephone box ever since. The Doctor 'borrowed' it when he left his home planet of Gallifrey. The TARDIS is an advanced piece of technology but sometimes it can be unpredicatable. Many of the Doctor's adventures have begun when the TARDIS has landed in the wrong place - or the wrong time.

"We've overshot a little." - the Doctor
"...how much is a little?" - Sarah
"About 30 000 years." - the Doctor

The Jellybabies and the scarf are not technically pieces of technology but they are symbols of the Doctor's fourth regeneration. This Doctor always carries a bag of jellybabies and frequently offers them to people he likes. The unbelievably long scarf was made for him by Madame Butterfly and is constantly dragging along the ground. Over the years, it has been shot, used as a weapon or tripped the Doctor over at critical moments.