Sherwin was responsible for the idea of exiling the Doctor to Earth (which happens at the end of the Second Doctor's last serial, "The War Games"), a decision he took in an attempt to improve falling viewing figures, reinvent the programme and bring more reality to "Doctor Who" by basing it more on Nigel Kneale's "Quatermass" serials from the 1950s. For this purpose, Sherwin created the United Nations Intelligence Taskforce (UNIT) in "The Invasion" as an organisation that the Doctor could become allied to on Earth during his exile. UNIT became a prominent feature of "Doctor Who" throughout the Third Doctor's era, when "Doctor Who" was produced by Sherwin's successor, Barry Letts. Sherwin left "Doctor Who" after Jon Pertwee's debut serial, "Spearhead from Space".