- You may be looking for the Galaxy 4 episode, Air Lock.
An airlock was a device that permitted the passage of people and objects between two drastically different environments, such as between a pressurised space and a vacuum or between an underwater and dry environment. The lock usually consisted of two airtight doors in series which did not open simultaneously. This allowed the intermediate chamber to change from one environment to the other.[source needed]
To prevent Kirksen from blackmailing the First Doctor into returning the ship to Kembel, and the Daleks, Katarina sacrificed herself by opening the airlock door on Mavic Chen's spaceship, Spar, expelling Kirksen and herself into the vacuum of space and killing them both almost instantly. (TV: The Daleks' Master Plan)
The Doctor's TARDIS had an airlock which opened on the roof of the police box. (COMIC: The Underwater Robot, Peril at 60 Fathoms) However, on another occasion the Doctor claimed that the TARDIS didn't have an airlock. (PROSE: When Starlight Grows Cold)