While the Brigadier investigates a loss of contact with Mars Probe 7, the Doctor realises signals coming from the probe are of extra-terrestrial origin.

UNIT retrieve the returned Recovery 7 but then Carrington hijacks it while the Doctor and Liz try to decode the signal he sent.

Carrington is holding the astronauts at a secret location but when he takes the Doctor, Liz and the Brigadier there they find they have been kidnapped.

Liz has been captured by Reegan's men and the Doctor begins to suspect Taltalian is more involved than he appeared.

The Doctor decides to take Recovery 7 back into space to search for the missing astronauts but Reegan sabotages the launch.

The Doctor speaks with the alien captain and learns the astronauts sent to Earth were ambassadors. However, before he can alert the authorities he is kidnapped by Reegan.

Carrington is planning to expose the aliens on live television and call for an interplanetary war and the Brigadier must rescue the Doctor and Liz in time to stop him.

Earth, the near future. When an archaological dig at the village of Devil's End goes disastrously wrong, an ancient power begins to revive. Can even the Doctor withstand the power of the Daemons? And just what is the local vicar up to in his spare time?

With the Doctor frozen by the energy release from the barrow, Jo calls Yates and Benton for help, but when the Brigadier tries to join them he finds the village surrounded by a heat barrier.

The Doctor explains to his friends that the Master is trying to gain the power of the last of the Daemons, a race who influenced Earth's development, while the Master brings the rest of the village under his control.

The Master orders Bert to set a trap for the Doctor at the village while a concussed Jo makes for the cavern.

The Master decides to sacrifice Jo to Azal and when the Doctor and his friends try to intervene they come under attack from Bok.

The Doctor and Jo visit the Master at his island prison where the Doctor becomes curious about a number of ships sinking in the area.

The Doctor and Jo are trapped on the fort by a Sea Devil while the Master convinces Trenchard to help him steal some electronic components.

The Doctor is held prisoner by Trenchard while the Master attempts to contact the Sea Devils.

The Sea Devils attack the island prison and rescue the Master, prompting the Doctor to venture underwater to try and contact them.

The Doctor is taken to the Sea Devils' underwater colony where he tries to make peace with the creatures, only for the navy to attack the base.

The Sea Devils take over the naval base and the Doctor is forced to help the Master find a way to revive their colony.

The planet Peladon, some time in the future. Mining disputes threaten the peace of the planet, and as the Doctor and Jo arrive, it seems that the ghost of Aggedor walks once more...

The Doctor turns the Aggedor spirit against the Ice Warriors to aid the Peladonians in their battle but Eckersley attacks him with the security system.