'Earthshock' (4 Parts) Story Code '6B' by Eric Saward

The TARDIS materialises in a cave system on Earth in the 26th century, and the Doctor and his friends investigate the dinosaur fossils around them. Meanwhile, a military team led by Lieutenant Scott is investigating the murder of members of a palaeontological expedition led by Professor Kyle. Encountering the troops, the Doctor and his friends are blamed for the deaths, but then they and Scott's team are attacked by two vicious androids. The Doctor realises that the androids are guarding a gigantic bomb, hidden in one of the caves. The soldiers destroy the androids, enabling the Doctor and Adric to defuse the bomb. The Doctor uses the TARDIS to trace the bomb's activation signals, and he takes his companions, Scott, and Kyle to a space freighter that is approaching Earth. The planet is on red alert due to an interstellar conference that is being held there, but the ship's captain, Briggs, has been given clearance to deliver her cargo on time. Briggs accuses the Doctor of the mysterious deaths of her crewmembers, but it soon becomes apparent that the real enemy is the Cybermen, who, together with a traitorous officer named Ringway, have replaced the ship's cargo with an army of Cybermen. The Cybermen take over the ship, planning to crash it into the Earth in order to destroy the conference; they lock the ship on course, and the Cyberleader forces the Doctor to take him, Tegan and Nyssa on board the TARDIS so that he can witness the destruction from a safe distance. Adric is left on board the ship with Briggs and Scott, who is able to destroy the remaining Cybermen. Adric attempts to defuse the bomb, but his interference causes the ship to plunge into a time spiral, dragging the TARDIS with it. The Doctor uses Adric's gold mathematician badge to disable the Cyberleader, and then destroy the silver giant with his own weapon. Scott and Briggs leave the ship in an escape pod, but Adric elects to stay on board to try to stop the bomb. The Doctor, Nyssa and Tegan look on in horror as the ship smashes into the Earth, killing Adric, and causing the explosion that wiped out the Dinosaurs.

Peter Davison (The Doctor), Matthew Waterhouse (Adric), Janet Fielding (Tegan), Sarah Sutton (Nyssa), James Warwick (Scott), Clare Clifford (Kyle), Steve Morley (Walters), Suzi Arden (Snyder), Ann Holloway (Mitchell), Ann Clements (First Trooper), Mark Straker (Second Trooper), David Banks (Cyber Leader), Beryl Reid (Briggs), June Bland (Berger), Alec Sabin (Ringway), Mark Hardy (Cyber Lieutenant), Mark Fletcher (First Crew Member), Christopher Whittingham (Second Crew Member)

Produced by John Nathan-Turner
Directed by Peter Grimwade

TX:
Part 1 - 9th March, 1982 @ 6.55pm - 7.20pm
Part 2 - 10th March, 1982 @ 7.05pm - 7.30pm
Part 3 - 16th March, 1982 @ 6.55pm - 7.20pm
Part 4 - 17th March, 1982 @ 7.05pm - 7.30pm

Notes:
*Featuring the Fifth Doctor, Adric, Nyssa and Tegan

*Episode Four is the series' only instance of the end credits rolling in silence

*Working title: 'Sentinel'

*This story was repackaged in 2023 in omnibus format under the banner of 'Tales of the TARDIS'; it was released on the BBC's iPlayer in the UK only, and had linking scenes featuring Peter Davison and Janet Fielding portraying 'memory versions' of their original roles: 'The Doctor and Tegan meet again and remember their terrifying adventure against the Cybermen to save the Earth… and the friend they lost along the way.' [Note: I don't consider this canon, so have not included it in the main story timelines]