The Doctor stole the TARDIS from a Time Lord museum when he began his travels. It was described as being a rather old "Type 40" model, whereas the best version is the "Type 70" used by the upper class Time Lords. The TARDIS has a device called the "chameleon circuit" which is supposed to change its shape on the outside to allow it to disguise it. In the very first story, this device stopped working and left the TARDIS stuck in the shape of a police box.