The TARDIS materialises on the planet of Oblivion. Despite its name it is actually a very pleasant place, half ocean and half forest. The Doctor explains to Gisella that its name comes from the fact that people go there to forget their worries. The planet’s real name is Ursulonamex and its living spaces are ‘Cities of Green’ made in the clearings and out of trees themselves. Some trees are hollowed out and provide accommodation, others have houses built in the branches and are connected by wooden walkways and bridges.

As they walk towards the nearest City of Green the Doctor smells burning and they see that the habitation is now a charred remnant. Gisella and the Doctor fight off a large, terrified animal before entering the ruined forest. The green-skinned natives give them a muted welcome and tell them that the destruction came from above in a rain of fire. He says that the attack was from ships in the sky but nobody saw them, though he thinks the space stations might know more. The Doctor and Gisella return to the TARDIS in silence. Back at the console the Doctor guesses that the destruction was caused by someone who wanted to keep their meeting with the Darksmiths a secret. To find the culprits they travel up to one of the space stations to see if there is any record of what happened.

They land in the docking bay of Orbit Station 3. Everywhere is chaos: debris litters the floor, cables are hanging loose from the ceilings and systems are offline. The Doctor notices that a ship of a type he doesn’t recognise is docked with the station. They pass into the main part of the station and ask the damage repair crews if they can be of any help. They are told that the Dravidians have arrived to help. The Doctor asks Chief Engineer Tyrall to tell him what happened and is given the news that Orbit Stations 1, 2 and 4 were destroyed but Station 3 survived the attack because it was round the back of the planet, though it did take a lot of damage. The engineer says that data from the attack was recorded but the damage to the systems means it hasn’t been analysed yet.

They head to the main control room where the Doctor’s psychic paper convinces Commander Sarla that they are from the Galactic Bureau of Investigation. Behind him is a giant insect, a little like a ferocious grasshopper that he introduces as Hive Captain Mantis from the Dravidian ship. Mantis says that his ship was in the sector and picked up a distress call. It has come to offer what aid it can to the Station and the crew, Mantis and two others, are working on the life support systems. The Doctor remarks that this is odd, as the life support seems to be working, but the captain says he is improving efficiency.

The Doctor asks to see the data systems and is led away by Lieutenant Jagellan. She takes them to a control room and the Doctor uses the opportunity to ask her about the Dravidians. She says they have only been aboard for a few days and have been working on the oxygen cycle since they arrived. The Doctor comments that, if anything, there is too much oxygen now. He tells Jagellan to consider what the Dravidians are really contributing.

After they are left alone the Doctor tells Gisella that the Dravidians are a nasty race who are known for their habit of stealing whatever they can but he is prepared to give this crew the benefit of the doubt. He accesses the data systems and copies what he finds onto a data sphere to take to the TARDIS for processing. Much of the information is damaged or incomplete but he thinks the TARDIS might be able to fill in some of the blanks.

They decide to go to the canteen in search of a cup of tea and pass a Dravidian hard at work. Another two are in the canteen. This strikes them as odd, since they were told by Commander Sarla that there were only three Dravidians on the ship and they have already seen four. Gisella wonders what else they are lying about.

They drop the data sphere into the TARDIS for analysis and then split up to snoop around the Dravidian ship. Gisella makes her way through the ship’s dimly lit corridors, hung with creepers and roots and control panels that resemble tree stumps and fungus. On the way she sneaks past a number of Dravidians. She arrives at a huge chamber containing a vast cone made up of hexagonal cells. Inside each cell is a Dravidian – there are thousands of them – waiting to hatch. She turns back, intending to warn the Doctor when she is discovered.

She runs blindly through the corridors, pursued by several insects. At last she bumps into the Doctor and he leads her back to the door he came in by. He locks the door behind him to keep the guards at bay while they make it to the main airlock. Then they head through the Station to warn Commander Sarla of their findings.

Jagellan and Sarla are amused by the news that there are more than three Dravidians. They call Captain Mantis over to explain and he says that he is sorry if he gave the wrong impression; in fact there are many more than three of them but the others have been left on the ship to care for the hive. If any others can be spared they help out with the Station repairs when possible. Gisella says that the crew on the Dravidian ship tried to kill her but Mantis says they were merely protecting the hive and if the Doctor wants to see around his ship he only has to ask. The Doctor apologises, noting that the Captain is still busy with the life support systems. As he and Gisella leave the control room the Doctor is approached by another insect. It warns him that if he interferes with their plans then Captain Mantis will have him killed and fed to the Dravidian young.

While the Doctor and Gisella are wondering what the Dravidian plan is and what to do about it they are called to the briefing room by Jagellan. There, Commander Sarla shows them security film of the Dravidian’s threat to the Doctor. He says that it was the Doctor he was concerned about, thinking that he was the trouble causer, but now he realises that it is the insects that are the danger. Sarla and Jagellan say they have been suspicious of the amount of oxygen the Dravidians are generating and the power that the Dravidian ship is draining out of the Station.

The four of them enter the Dravidian ship. The corridors are much brighter now. In the hatchery they see thousands of Dravidian young forcing their way out of their cells. Hive Captain Mantis enters the chamber and makes a speech to the young insects. He tells them that the attack on the planet was recorded by the station and that the data can be used to make a weapon of similar power. This is what the Dravidians want. They are going to storm the station and steal the data to make copies of the weapon. After that they will destroy the Station so that their secret is safe.

The four listeners return to the Station and order the crew to barricade themselves into the control area. As they do so Chief Tyrall sees a ship approaching on the screens. It is a design that the Doctor recognises: Dreadbringers.

The Dravidians rampage through the corridors of the Station, damaging everything in their path. The Doctor has a plan. He knows that massive insects need massive amounts of oxygen to stay active so he works on the life support systems top reduce the supply. As he does so the door is burst open and Mantis enters. Gisella grabs a fire extinguisher and covers the creature in carbon dioxide, bringing him to his knees. The Dravidians behind the captain flee in terror. The Doctor waits for Mantis to begin to recover before telling him that the oxygen levels have been reduced and that this is the Dravidians’ only chance to escape.