The TARDIS materializes on Earth in the twenty-sixth century, where the Doctor assists a squad of troopers investigating the disappearance of an archaeological expedition. The Doctor discovers a large explosive device concealed in a cavern, when the group are attacked by two androids. The androids are destroyed and the Doctor manages to disable the bomb. The Doctor pilots the TARDIS to a freighter bound for Earth - having identified it as the source of the radio signal controlling the bomb. It transpires that the bomb was the work of the Cybermen who have a huge army concealed in the freighters hold. The Cybermen plan to crash the freighter into the Earth, however Adric who is still aboard, inadvertently causes the freighter to travel back in time, where it collides at a time when mankind did not yet exist.
Sentinel was a story hastily written by script editor Eric Saward to replace a problematic script commissioned from writer Christopher Priest. Priest's original script for The Enemy Within had to be rewritten when John Nathan Turner announced that Adric was to be written out of the series at the conclusion of the story. Priest carried out the necessary amendments, but neither Nathan Turner nor Saward were satisfied and further rewrites were requested, Priest refused to do so unless he received an extra fee, at which point Nathan Turner decided to abandon the story altogether.
Because of the poor audience ratings that season eighteen had received, Nathan Turner and Saward decided that it was time to resurrect an old enemy. Both men felt that the Daleks had lost their impact with audiences, and so they decided that it would be the turn of the Cybermen to make a re appearance. Nathan Turner ordered that the storyline of Sentinel, now called Earthshock was to be kept a closely guarded secret to preserve the impact of the Cybermens return, to this end the public viewing galleries at BBC Television Centre were closed, and an offer of a Radio Times cover was declined.
The task of redesigning the Cybermens costume fell to designer Dinah Collin. Collin discarded the old rubber diving suits that had been used in previous years, electing instead to use hard wearing military G-suits. It was John Nathan Turners idea to incorporate a transparent jaw into the helmet; he felt that I would help convey the fact that Cybermen were once human themselves. Collin wanted to do away with the traditional 'handles', but Nathan Turner vetoed the idea, felling that they were integral to the design, another proposal, this time for their hands to be left bare as in The Tenth Planet was similarly abandoned.
After a slow start to Peter Davison's opening season, Earthshock brought a welcome injection of pace. Thanks to Director, Peter Grimwade's eye for detail, Earthshock is a very slick looking production. Where other Directors might have settled for less, Grimwade insisted on perfection and he was renowned for making actors redo scenes until he was happy with them, the positive results of this policy can be seen throughout the story.
In the first episode, the pace at the beginning is slow, serving only to introduce the audience to the main characters, the real action would not begin until the conclusion of the first episode, when we got our first surprise glimpse of the newly designed Cybermen, a truly memorable cliffhanger. From the onset of episode two, events start to gather pace, the action moves to the freighter, where we find out what the Cybermen are planning, and discover, in scenes reminiscent of The Tomb of the Cybermen, that they have an army hidden in the hold.
This was David Banks first of many outing's as the Cyberleader, and he does a terrific job. Setting aside the premise that Cybermen are supposed to be devoid of emotions, Banks portrays his Cyberleader as both pompous and arrogant, and indeed there is some terrific dialogue between him and the Doctor in the final episode.
The most memorable part of Earthshock comes at the finale of the story, where Adric elects to stay aboard the doomed freighter in an attempt to prevent it from crashing into the Earth. Aside from the demise of Katarina during The Daleks Master Plan in 1965, this is first time in Doctor Who history that a main companion is killed off. The manner in which Earthshock ends compliments this story well. No doubt it was a particularly bitter sweet moment for Matthew Waterhouse, who despite his lack of popularity with fans, got to have one of the best final scenes of any companion.