Outer plasmic shell

An outer plasmic shell, often simply called a plasmic shell, was a TARDIS exterior. In a perfectly functioning TARDIS, the chameleon circuit picked a plasmic shell suitable to the time and place in which the ship landed. A TARDIS' shells were stored in the shell room; if the room had no suitable shells, more were created and stored as well. (PROSE: The Little Things) Lord Roche was forced to use a non-terrestrial aeroplane as his shell when he didn't have any terrestrial designs in his shell room and didn't feel like designing any. (PROSE: The Suns of Caresh) The theory in the TARDIS Handbook was that the plasmic shell was driven by the chameleon circuit. (TV: Logopolis) When a TARDIS died or was dying, its final plasmic shell might grow to enormous size due to leakage of its dimensionally transcendental interior. (TV: The Name of the Doctor)

Due to a malfunction in the chameleon circuit, the Doctor's Type 40 TARDIS was stuck with a police box plasmic shell after leaving 1963 London. (TV: An Unearthly Child) Apart from the Sixth Doctor's brief and somewhat limited success in repairing it (TV: Attack of the Cybermen), it retained that shape up through it's final resting place as the Doctor's tomb on Trenzalore. (The Name of the Doctor)

Among plasmic shells used by the Doctor's TARDIS before the malfunction were: a wooden kiosk, (AUDIO: Quinnis) a boulder, (PROSE: Frayed) a shed (PROSE: The Price of Conviction) and, finally, a post box. (PROSE: The Little Things)

The Tremas Master used his chameleon circuit to bond his TARDIS' plasmic shell with the Third Doctor's. (PROSE: The Quantum Archangel)

Later, more sentient TARDISes could decide what to change into all by themselves. (PROSE: Alien Bodies)