We deal with the odd... the unexplained. Anything on Earth... or beyond."Spearhead from Space" was the first Doctor Who story to be broadcast in colour, as well as the beginning of a long story arc (ending with "The Three Doctors") that takes place exclusively on Earth. Budgetary restraints now meant that the setting would be "contemporary Earth" for the forseeable future, with the Doctor's "exile" being a case of Real Life Writes the Plot. With the Third Doctor being confined to life with UNIT, the show's atmosphere changed completely and started drawing much inspiration from that other famous Sydney Newman show: The Avengers.
— The Brigadier sets out UNIT's mission statement
The TARDIS materialises (in colour, no less!) and a tall, white-haired man steps out and collapses.UNIT troops arrive and find the man, taking him off to hospital, where he spends some time in a coma. Brigadier Lethbridge-Stewart arrives and, after hearing that the man has two hearts and was found near a police box, concludes that it's probably the Doctor. When the Doctor wakes up — still with his previous personality — he's not at all happy at having regenerated, but has to admit that his new face is rather dignified. One gratuitous shower scene later, he's stolen some clothes and a car (from the "Doctors only" car park) and is on his way to UNIT, where scientist Liz Shaw is trying to make sense of the mystery of the day. An employee of the local plastic factory is claiming that he's seen a walking mannequin. And there's a meteorite shower on.The new Doctor is still suffering from amnesia (caused by both his regeneration and the Time Lords having removed large chunks of his memory) and tries to get used to his new personality while he investigates the strange meteorite shower. He discovers that the meteorites are in fact hollow globes containing the Nestene Consciousness, another disembodied alien intelligence (see "The Abominable Snowmen"), which uses plastics to achieve its ends. In one classic scene, shop dummies come to life, smashing through the shop windows and firing guns concealed in their hands, spreading panic — but the Nestene Consciousness's real plan is to create plastic replicas of world leaders and take over the Earth.The Doctor and Liz manage to repel the Consciousness back into space and the Doctor agrees to sign on as UNIT's "Scientific Advisor" for the duration of his exile, signing the paperwork as Dr. John Smith. In exchange, UNIT will give him technical facilities to try and repair the TARDIS (which has been deactivated by the Time Lords), and the use of a vintage car just like the one he had earlier appropriated from the hospital.