The TARDIS lands on the cold wastes of ice at the northern pole of a planet. It has materialised at an odd angle but when the Doctor steps out he jumps up and down a few times to make sure the ice is solid. Satisfied, he sets off to get his bearings. Very soon he realises that the place he is looking for is not there. He had materialised about half a mile away from the co-ordinates of his destination so that he wouldn’t attract attention. Now he realises that the energy source he traced is half a mile beneath his feet at the bottom of the ocean. He turns and makes his way back to the TARDIS but before he can reach it the ice cracks in front of him and a large creature with too many tentacles to be an octopus starts to slap the ice near him. It then grabs hold of the TARDIS and tries to pull it through the ice. The Doctor nimbly runs along one of the tentacles to the door of his ship and leaps in. He dematerializes, leaving the puzzled creature grasping thin air.
The TARDIS materialises in a gloomy maintenance area of an undersea base. Water seeps in everywhere and the pipes are corroding. The Doctor makes his way through a bulkhead door, carefully closing it behind him, and meets a small old man in grimy overalls. This is Hank, the man who is responsible for the maintenance of the undersea scientific base. The Doctor uses his psychic paper to let Hank think he is Doctor Smith and that he has turned up to perform a surprise inspection.
Hank doesn’t seem that surprised by the inspection and leads the Doctor to the rather shabby common room. The rest of the crew is present there. They comprise of Captain Strova, a tall woman in military uniform, Privates Lenk and Rodoff, and a girl who seems to be about eleven years old. She introduces herself as Gisella and says she is in charge of the base. The Doctor tries to pretend he isn’t surprised by this. Strova reminds him that, as he ought to know, Gisella suffers from Adulescentia Perpetuus, a condition that means she never gets any older. The Doctor jokes that Gisella is used to being treated like a child and she responds by saying the Doctor must get fed up of people treating him like an idiot. He offers her some ice cream and a game of tiddlywinks, which eases the tension, and Gisella remarks that the Doctor is used to playing the idiot as an act but she can tell he isn’t one at all.
Over a mug of tea Strova reminds the Doctor that the base was built one hundred and fifty years earlier when the planet, Flydon Maxima, was still inhabited. It was meant to learn from the effects of global warming. The problem was that the pollution that started the process was too far advanced and the warming precipitated a catastrophic ice age that meant the majority of the planet’s inhabitants left for Flydon Minima. While they talk the Doctor examines some lumps of rock that are being used to decorate the room. Gisella says that they are probably volcanic sediment that has cooled rapidly in the icy sea. Rodoff says he brought most of them back when he was surveying a cave near the base for surviving sea life. The Doctor describes his narrow escape from a local life form and the crew members identify it as a Blaska. Strova says that the Blaska have a reputation for being aggressive but Lenk adds that they have got a lot worse recently and seem to be waiting close to the base so that they can attack anyone who leaves. Rodoff says that he was nearly trapped by one as he left the cave with the lumps of rock.
They take the Doctor on a tour of the base, starting with the machine room. The technology there is unmistakably a product of the Darksmiths. When the Doctor asks where it came from Gisella merely says that it was donated by someone called Varlos. Before he can inquire further the Doctor is interrupted by a message from Rodoff. He says he is in the observation gallery with Lenk and that there is a problem. The rest of his message is cut off by Hank grabbing the microphone and saying that the Blaska are attacking. This is followed by the sound of a large crash.
Strova and Gisella lead the Doctor on the long and time consuming journey through the base, opening and closing each water-tight door as they go. The observation gallery is on top of the base and has a huge domed glass ceiling. The Blaska assault has put a small crack in the glass and when one of the creatures launches itself at the glass there is a sound like thunder. The impact widens the crack and water starts to drip in. A second Blaska approaches at speed and everyone races for the door. As the Doctor steps out, last to leave, there is another impact and the glass shatters. The Doctor locks the door just in time. There is a thud against the door and Gisella points out that any creature capable of getting through three metres of finite-glass won’t find the doors a problem.
Gisella says they have to get to Level 3, the Green Area which is better protected and has a freight capsule to the surface. As they run down the corridors the Blaska are close behind and so is the water pouring into the base. With each door the danger closes in. the Doctor uses a computer terminal to open some of the doors and divert the water through to other parts of the base, hoping it will draw off the attack. He asks why the Green Area is so secure and is told that it has blast walls because the nuclear reactor is housed there.
Once they arrive a quick check of the display screen shows that the Blaska are still inside the base, heading towards the living quarters. A collision alarm tells them something massive is heading towards the base. It hits with tremendous force. Reading the analysis of the object the Doctor realises that it was ice from the surface. He surmises that something heavy hit the ice from above and sent it down.
The TARDIS is on the other side of the base so the only way the Doctor can get to the surface to investigate is up the freight capsule. Gisella goes with him. She is puzzled because he doesn’t recognise the capsule; it should have been the way he arrived. On the way up, the Doctor questions Gisella. He thinks she knows more about Varlos than she is letting on. Also he says he has never heard of Adulescentia Perpetuus. He says he needs her help to find Varlos. At that moment the capsule arrives on the surface.
The doors open to show them a view across the ice. A huge stone cathedral is standing there now, a Darksmith spaceship. Pouring from it are lines of heavily armoured soldiers – Dreadbringers. The leading soldier aims a blast weapon at the capsule so they close the doors again and head back down. When the Doctor tells Gisella that the Dreadbringers are the enforcement arm of the Darksmith Collective she pretends that she has never heard of either before blurting out that she thinks they have come for her.
They arrive in the Green Area to be told that the Dreadbringers are dropping through the water in pressure capsule. An explosion on the hull indicates that the soldiers are already blasting their way through to the inside. As they put on breathing apparatus the explosions grow closer. They start to make their way out up some service stairs as the Doctor tries to get everyone to the TARDIS. Behind them a wall explodes and Dreadbringers pour in. They only just get out in time and seal the door behind them.
They make good progress and Hank says that if they cut through the art gallery they will be almost in the storage area. The Doctor wonders why they have an art gallery and is told that is what they call the area where they keep most of the volcanic sediment that they found. They were hoping to sell the objects when they returned to Flydon Minima. As they approach the art gallery they find themselves trapped in a tunnel between a Blaska and the pursuing Dreadbringers. A shot from one of the soldiers narrowly misses them and hits the wall, weakening it. The Doctor’s screwdriver pops rivets out of a panel so that they can escape into the art gallery while the Blaska turns its attentions to the Dreadbringers. Inside are a hundred pieces of the coral-like volcanic rock. The Doctor produces a stethoscope and proclaims that the rocks have heartbeats; they are Blaska eggs. As the parent Blaska is so close outside the eggs start to crack open.
The Dreadbringers burst into the art gallery and the Doctor tosses one of the eggs to the first man in. he catches it in surprise and is suddenly attacked by the Blaska in the corridor. The Doctor rolls more eggs at the incoming soldiers and a battle is resumed between the giant creature and the soldiers. Using the confusion as cover, the Doctor orders the others back into the living quarters and tells them to seal themselves in. He says he is going to open the outer airlock doors to let the water in and flush away the Dreadbringers. By the time they have recovered themselves he will be gone and so will the object they are seeking. Gisella tells him he needs her to open the doors because the system needs to recognise her eye pattern.
He takes her into the TARDIS and uses the console to access the base’s security systems. A small camera near the view screen accesses Gisella’s eye pattern and the Doctor starts the sequence to open the airlocks. Dreadbringers arrive outside the TARDIS and begin shooting the doors but the Doctor calms Gisella by telling her that nobody can force their way in.
Icy cold water crashes through the base, washing away Dreadbringers and allowing the Blaska to collect their children and leave. The Doctor shows Gisella the Eternity Crystal and tells her that this was the Dreadbringers’ target. He says he needs to find Varlos to find out how to destroy the Crystal and asks for Gisella to go with him to help. A quick radio call to the living quarters tells them that the others are safe.
The Doctor prepares to dematerialize the TARDIS. He asks Gisella where to go to find Varlos. She tells him that Varlos retired to Earth to hide from the Darksmiths. He was terrified of what might be done with the Crystal and had hidden it until he could find a way to destroy it. In the same way he hid Gisella on Flydon because she was his daughter.