The Doctor visits a war museum that is under attack by guerrillas. He grabs a disc and uses it to read a display that informs him that Earth is a target.
Back in the TARDIS, Sarah wants to go somewhere for Christmas. The Doctor takes her to Papua New Guinea. It is very swampy and full of leeches. They come across a crashed spaceship mostly sunk in the mud. There are human and animal remains in the ship. The Doctor explains that there was no pilot, but there is something out there.
Suddenly the ship is under attack from a bunch of tribesmen. The ship is almost breached when there is silence. They peek outside and see a man-like reptile wearing sophisticated armour. The Doctor uses the disc to shield himself and Sarah from the reptilian's spear. Sarah notices that the disc and the armour have the same markings.
The Doctor tries to tell the reptilian that its orders are to shut down, but it doesn't believe him. It grabs the Doctor, who inserts the disc into a slot on the armour. The armour detaches itself from the reptilian, revealing a normal Earth crocodile.
The Doctor explains that the armour was created by the Nervoids, who send it across the galaxy to conscript the dominant species of a planet for battle. It takes over the individual, even giving it sentience.
They head back to the TARDIS, on the way being chased by the tribesmen who had run off before. As they make their escape, the armour starts to connect itself to the Doctor. He frees himself and the armour falls into a waterfall.
Back in the TARDIS, the Doctor and Sarah wish each other a Merry Christmas. Meanwhile, the armour attaches itself to a crayfish, using it until the batteries die.
- Sarah mentions South Croydon.
- The Doctor mentions Magellan Five.
- Sarah thinks UNIT should take care of the craft.
to be added
- The Fourth Doctor tells Sarah that he often pops out of the TARDIS while she is asleep. The Eleventh Doctor would also leave the TARDIS for adventures while his companions Amy and Rory were asleep. (TV: Bad Night)