Outer plasmic shell

An outer plasmic shell, often simply called a plasmic shell, was a TARDIS exterior.

Characteristics
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)

The Second Doctor boasted that the TARDIS was made from the "hardest materials in existence", making it near impervious to harm. (COMIC: The Monsters from the Past) Indeed, firepower from Dalek gunsticks were unable to penetrate the plasmic shell. (AUDIO: Beneath the Viscoid)

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)

When the Doctor's TARDIS entered siege mode, its exterior took the shape of a cube etched with Gallifreyan writing. (TV: Flatline)

The Doctor's TARDIS
Among plasmic shells used by the Doctor's Type 40 TARDIS were: a boulder, (AUDIO: The Beginning, PROSE: Frayed) an Ionic column and a sedan chair, (TV: An Unearthly Child) a wooden kiosk, (AUDIO: Quinnis) a shed (PROSE: The Price of Conviction) and a post box. (PROSE: The Little Things) However due to a malfunction in the chameleon circuit, caused by the Eleventh Doctor, (COMIC: Hunters of the Burning Stone) the Doctor's TARDIS became stuck with a police box plasmic shell after leaving 1963 London. (TV: An Unearthly Child)

The shell of the Doctor's TARDIS had deteriorated by the time Romana I travelled with the Doctor. After a near miss by a missile in Atrios orbit, (TV: The Armageddon Factor) she designed a routine to strengthen it, though the Doctor disabled it as he abhorred tampering. An action he regretted when a missile was aimed at the TARDIS in Proxima Major orbit. (AUDIO: The Dalek Contract)

For a short period of time, the Sixth Doctor managed to repair the chameleon circuit, resulting in the outer shell taking the appearance of a stove, an organ and a doorway. The shell eventually returned to the shape of a police box. (TV: Attack of the Cybermen)

In 1854, the TARDIS was struck by cannon fire which displaced the outer shell. This triggered the Hostile Action Displacement System, causing the TARDIS to retreat to the Time Vortex where it grew a new shell. The new shell was still a police box, but was coloured white. The remains of the old shell were used as fire wood by soldiers. (AUDIO: The Angel of Scutari) The outer shell later regained its normal blue colour. (AUDIO: Black and White)

During the Last Great Time War, the TARDIS’ outer shell showed signs of wear and tear. (TV: The Night of the Doctor, The Day of the Doctor)

By the time of the Tenth Doctor, the outer shell appeared to have deviated somewhat from the original design of a police box as DCI Billy Shipton noted that it’s windows were the wrong size. (TV: Blink)

Whilst confronting the Prometheans, the Eleventh Doctor learnt that the police box shape of the TARDIS’ outer shell had become part of humanity’s race memory due to his many visits to Earth, even speculating it had paradoxically influenced the design of police boxes in the first place. He used this to his advantage to free humanity from the Prometheans’ control. (COMIC: Hunters of the Burning Stone)

In an alternate timeline, the TARDIS’ outer shell retained the police box shape up through its final resting place as the Doctor's tomb on Trenzalore, though it grew to an enormous size due to a size leak. (TV: The Name of the Doctor)

When the TARDIS’ power was drained by the Boneless the outer shell shrank, causing the Twelfth Doctor to be trapped inside. It became small enough that Clara Oswald was able to fit it in her handbag. After she tricked the Boneless into returning power to the TARDIS, the outer shell regained its usual size. (TV: Flatline)