Field Medic Harrison squelches across a misty, muddy battlefield on Rescension. He avoids a vehicle reversing and tells somebody to mind out of the way. He is cold, wet and tired. Soldiers busy themselves around him. He tells the stranger that an Akwabi bombardment is due. The stranger introduces himself as Nash, a journalist embedded with the platoon. Harrison says he guessed that and refuses his handshake. He says that General Brudge mentioned Nash earlier. He tells the journalist to unload the supply vehicle. Another supply vehicle moving nearby does not quite disguise the sound of the TARDIS landing. In the truck Donna steps out of the TARDIS and sniffs the air.

The Doctor follows her out and shuts the door. Out of the tarpaulin they can see that it is a bright day. They are surrounded by crates. Donna complains about the smell which the Doctor identifies as uniforms, Weapons lubricant and sweat. He uses his screwdriver to unlock one box and sees isomorphic hand guns. Just then the flap at the back is opened and a young man looks at them in some surprise. The Doctor uses his psychic paper to introduce them as Doctor McCoy and Captain Kirk. The young man accepts this surprisingly quickly. They exit the truck and see bright dense fog and soldiers tramping through the thick mud, holding futuristic weapons. The Doctor is unsympathetic to Donna’s plight, stuck knee deep in the mud.

The Doctor recognizes the troops as part of the Second Great and Bountiful Human Empire. Three soldiers (one a woman) unload the boxes. Harrison arrives and hands her some capsules, saying they are for immunization. He dashes off. The Doctor lifts Donna out of the mud, leaving her boots behind. A squealing from above alerts the Doctor to an incoming bombardment. Something lands in the mud nearby. Harrison arrives, looking to Donna for orders. Before she can respond he is shot dead. Soldiers around her shoot upwards. Creatures drop from the sky. At first Donna thinks the enemy is dropping ponies but the Doctor says that it is riders on horseback, suspended under parachutes that are falling around them. The Doctor tells her that they are Akwabi warriors. A dead creature falls nearby and Donna sees that it is not rider and horse, but a centaur. Other centaurs are brutally gunned down and the injured are slaughtered.

The surviving centaurs gallop off into the mist. The Doctor remarks that the battle was over too quickly before his attention is caught by the muddy transport vehicle. It is a hover truck and it drenches them in dirty water as it speeds off. The Doctor is aghast that the TARDIS is on board. Jumping onto a motorbike, he gives chase. Soldiers dragging a stretcher race by. Donna prepares to help when she hears sobbing. It is the young dispatch rider whose motorbike has been taken. He is clutching a corpse and is bleeding heavily. Donna prises him away from his dead friend.

Donna sees the woman soldier, Merriam, looking at the boy. Following her gaze she sees the boy’s outline blur. He grows taller and broader, develops a cloak and seems to look like a giant, upright beetle. One of the stanchions she has noticed on the battlefield suddenly rises up. Donna realises that it is the metal leg of something very tall, obscured by the low cloud. It stamps down on the beetle, crushing it. The metal foot now stamps down at Donna. She is pulled to safety by Merriam whose leg is caught under the metal foot. Donna pulls her to safety as the metal leg stamps off into the distance. She realises that the soldiers around her are nothing more than a disconsolate bunch of kids.

One of them salutes her, referring to her as ‘Captain Kirk’. He is a sergeant. He asks for her identity papers. She says they were lost in the skirmish and happily hands authority to him. Nash speaks to her, introducing himself, and recognizes her as an imposter. Before he can say more the sergeant approaches. He orders them to get into the surviving troop carrier, and to help the wounded aboard. Donna wonders why the journalist does not give her away. She swallows an inoculation pill before tending to other survivors.

The Doctor rides up a rocky slope in pursuit of the truck, sensing the TARDIS with his emotional and mental bond to it. He sees that he is on a narrow mountain path with a steep drop to a stream below. He suddenly comes across the truck, halted by a fallen tree, and skids into the back. The collision mangles the tailgate latch. He runs to the side of the vehicle and finds two dazed Akwabi warriors in the cab – the truck has crashed into the tree. He squeezes past them into the rear. The TARDIS is there, but so is a female warrior. He kicks away her fallen crossbow but she grabs a revolver from a nearby box of weapons. He consciously relaxes and calmly says, “Isomorphic response.” This confuses her. He tells her that the gun is useless to her as it has not been primed for her use. He suggests that she drop it. Bewildered, she does so. Speaking persuasively he suggests that her legs are stuck to the floor. She realises that this is also true. He is about to let himself into the TARDIS when she punches him to the floor, reminding him that he forgot to disable her arms.

An hour later the transport vehicle sets off, with Donna helping the injured throughout. She reflects on how her time with the Doctor has changed her. She looks at how she adapts to this situation and wonders if she is more confident or more hardened. She is wearing one of the uniforms from the crates but stops short of picking up a gun. They drive through a desolate landscape, and Donna realises what this war has done to the Akwabi. Nash engages h er in conversation. Donna confesses that her name is Donna Noble, not Kirk.

She tells Nash that she and the Doctor crashed in a ‘shuttle craft’. He tells her that he is writing a profile on General Brudge, leader of the Pioneer Corps. Their task is to form a bridgehead for the main forces invading Rescension. Donna wonders why he is on the battlefield if he is meant to be profiling the general. Instead she asks him about the giant robot and the insects. He remains silent so she turns back to Merriam. The young soldier’s leg has been almost severed and Donna wonders if there is a chance that she will die.

General Brudge is hesitating in front of the door to surgeon Lanova’s door. He rules his troops with fear, yet the surgeon somehow fills him with a feeling that he needs her more than she does him. An orderly arrives with some water for the surgeon and the general takes it from him, reflecting that he hasn’t seen the surgeon eat for several weeks. He thrusts open the door without knocking. The room is sickly sweet to smell. The surgeon is plumper than he remembers, so she must be eating something. He tells her that there has been a delay in eradicating the enemy. She responds sa rcastically. He blames her for not providing the superior troopers she promised. She is turning injured and dying soldiers into stronger troopers on the operating table. He tells her that reinforcements are three months away. She blames him for falsifying reports to his superiors, suggesting that the campaign was progressing better than the true situation, for the lack of immediate reinforcement. He tells her that survivors of a platoon are on the way in but some are badly injured. She says her techniques can turn almost any injured soldier into a better one.

The truck pulls into the hospital. Donna helps disembark the wounded into a triage tent. She doesn’t want to leave Merriam but without identification papers her claim that she is a captain is not listened to. So she sits outside, waiting and seething in the sun. Nash takes everything in his stride. He tells Donna that he knows how to get in. General Brudge arrives. He is a tall thin, man who instantly commands everyone’s attention. He ignores everyone except Lt. Yovich, his second in command. He tells him that Lanova’s new batch of troops should soon be ready. Yovich says that not all will be ready. Brudge is furious and demands that the troops are m ade ready but he is told that some of the men have had adverse reactions to the treatment. The general calls them ‘Halflings’ (not pure blood humans). He is critical of ‘half-breeds from the outer solar system… trash’. Nash steps forward but the general brushes him away. Donna flares up at the General’s racism and asks where he was during the fighting. The general is aware of this mysterious new officer and calls her insolent. He asks if she is a natural born human. She jokes that she was a Caesarian birth, then says she cannot believe that the way his men slaughtered the Akwabi centaurs is war. The general tells her that it is pest control. With the interplanetary link down there is no way of verifying Donna’s credentials so the general says he will asses her credibility. Nash intervenes, saying that he knows she is an imposter, called Donna Noble.

Brudge says that she should be executed for wearing a uniform that she is not entitled to. Nash asks for a look at the medical facilities and an interview with surgeon Lanova in return for his information. The general agrees. The journalist is led away. Sergeant Chaco steps in on Donna’s part, saying how much courage and stamina she displayed in the aftermath of the battle. The general is pleased with Chaco and tells Donna she can earn her uniform by going out with the platoon. She asks to see Merriam, and is astonished when the young woman joins her almost immediately. There is no trace of her injuries and she is ready to return to the fight.

The Doctor wakes up in a cell. Humans in tattered uniform eye him suspiciously. They are hungry and demand food. They have already searched his pockets and found nothing. The cell door opens and admits the warrior who knocked the Doctor out. She hustles him upstairs into a ruined palace, shattered by the war. The Doctor sees what remains of a once elegant city through the gaps in the palace walls. Koblenza, the Akwabi demands that he look on what his sort have done, but the Doctor insists that he is not her enemy. They approach a less-damaged part of the building. The Doctor sees a room full of humming equipment. He slips in and sees a room full of scanners and monitors, manned by civilian Akwabi. He is dragged away by Koblenza. He is taken into a mirrored drawing room, laid out with maps. At the far end several Akwabi are gathered. The tallest is Commander Padova. They are gathered around the TARDIS, Padova holds out the TARDIS key and demands to know why it won’t open the door. Padova asks what sort of weapon is in the TARDIS. The Doctor tells him that it is a vehicle and that he is not a human.

Padova is displeased when the Doctor points out the shortage of Akwabi manpower and their obsolete technology. He asks if this is why they are attacking the human supply chains. Padova asks how the TARDIS works. The Doctor tells him that the key has an isomorphic response and the box is too small for Akwabi. He turns this conversation to the poor conditions of the human prisoners below. Padova replies that they are treated as well as the Akwabi can manage and that at least the Akwabi do not slaughter their prisoners. He pockets the key, tells the guards to throw the TARDIS into the garbage and return the Doctor to the cells. There he smells a sickly sweet smell from the cell next door. He begins to use the sonic screwdriver on the lock of the cell but it is dead-bolted. A clattering in the cell next door is followed by two male screams. One of the prisoners in the Doctor’s cell, Corporal Pine, demands to know what is going on out there before she convulses and stands erect. She blurs and changes into an enormous insect, clicking its mandibles menacingly. Most of the others change too. The Doctor recognizes them as Serfians. Akwabi guards stare in. The far wall of the cell explodes open with a blast from an adjacent cell. A bright light disintegrates two of the insects. Koblenza tells the Doctor that this rescue attempt will be foiled but he screams that this is not a rescue, it is a massa cre. The light is a focused plasma cutter. The door of the cell collapses and the Doctor forces past Koblenza into the corridor, He demands that the doors of all the cells be unlocked and the prisoners released. More light lances into the block and the Doctor sees that the all the humans in the adjacent cell have transformed into insects. Koblenza lunges in among them to avoid falling masonry. One of the insects is a young queen with a fat, slug-like body. She reaches for Koblenza’s hind quarters. Koblenza shoots the queen point blank but to no effect. Then a shaft of light from above hits the queen and dissolves her. The Akwabi guards, including Koblenza, and the Doctor race upstairs as the cell block collapses. In the dust they see a humanoid robot, 25 metres high, glittering in the sunlight. This is the cause of the destruction. The Akwabi fire their crossbows uselessly at it. Padova and his officers race across the courtyard at the Doctor seeking revenge. Koblenza confirms the Doctor’s shout that the robot is nothing to do with him. He says that something that large needs a sophisticated cloaking device. Too sophisticated for either the Akwabi or humans. Several Serfian insects escape from the rubble. The robot reaches out and crushes three with its hand. A fourth arrives next to the Doctor and the Akwabi. The robot lifts its foot and stamps down at them.

The Doctor hauls himself onto Padova’s back, shouting that the robot is after the insect. The Akwabi commander gallops away as his men shoot down the insect. The Doctor jumps off the centaur’s back and runs into the footprints of the robot, pointing his screwdriver upwards. Its shrill whistle causes the robot to stagger away into the city. The Doctor jumps onto the rubble to follow its progress. It is stumbling and disoriented as leaves. Chasing after it at full speed he sees it tumble across the river and fall into a power station.

When the Doctor arrives the substation is silent. The robot is sprawled across the rubble. A panel has fallen open in the robot’s back. The Doctor marvels at the complexity of its innards. It suddenly announces itself as a pest control robot and asks him not to reboot while it auto-repairs. The Doctor checks the robot’s recent history and recognizes that it originates from the same part of the galaxy as the Serfians. He realises that it was made by the Sharback Corporation of the Pettingard system and wonders what happened to the company that made it. The robot replies that the giants of Pettingard were wiped out after a viral plague broke out, spread by the Serfians. The virus did not kill them, however: it was the robots who eradicated them as they tried to get rid of the insects. The Doctor noticed that the robot was not bothered that other creatures got in the way of its attempt to kill insects in the Akwabi palace. The robot restores its systems and stands up, commenting that it must continue the program implemented by the masters and track down and kill all the Serfians. It stalks off across the landscape.

Donna holds a large but lightweight assault rifle. It is keyed to her palm print. The transport vehicle drops the soldiers off some way from a forest. The group of soldiers make their way into the silent forest. Merriam and the other soldiers swallow down their booster pills to inoculate themselves against the environmental hazards. Donna does not take her pill. They reach a clearing. Merriam complements Donna on her bravery in the previous battle and says she owes her life to her. Explosive coughs wrack the soldier. The soldiers scan the forest around them. Donna glimpses what she thinks is a man on a horse in the trees. Then she realises it is an Akwabi soldier. The other soldiers launch a volley of fire. Human and Akwabi soldiers exchange close-range fire. Some of the human troops have been deployed at the top of the trees while the others force the Akwabi into the clearing. The centaurs throw down their weapons.

Chaco’s men cock their rifles. Donna leaps into the clearing to prevent a slaughter. The Akwabi commander introduces himself as Padova. Donna notices that some of the humans have dropped their weapons and are standing still. They transform into insects. The centaurs stare in disbelief as the remaining humans flee. The massive robot suddenly towers overhead. It fires a massive cannon into the clearing, killing three insects, and stamping on another six. Donna suddenly sees the Doctor leaping off the robot’s ankle. Padova drives the Akwabi away but Sergeant Koblenza does not move, transforming instead into an insect. The robot stamps on her and then turns away into the forest.

Only Padova and one of his men remain in the clearing, along with the last few human soldiers. Donna insists to Chaco that they treat their prisoners within the appropriate rules. Chaco tells his men to fall back. The Doctor and Donna greet each other before he tells her that he hitched a lift on the robot’s foot after it mentioned a pheromone signal. The two of them carry Merriam between them as they follow the soldiers and their two Akwabi prisoners. Donna says that the robot arrived at the same moment as the insects appeared. The hover transport arrives at the edge of the forest and the soldiers and their prisoners board, followed by the Doctor and Donna. On the journey back the two of them discuss the robot’s whereabouts: he says the robot will re-appear when more insects hatch out. Donna reflects to herself that all the victims of the last battle transformed into insects. When the Doctor makes some disparaging remarks about General Brudge he is interrupted by Lt. Yovich who demands he show more respect. The Doctor drily notes that the General was absent from the conflict, whereas Padova was in the front line with his troops.

Padova says that his men will rescue him or seek revenge. This prompts the Doctor to say that this is how wars are propagated. He points out that the only way to change things is if the men of both sides team up against the biggest threat, the insects. He says that they are hatching from infested soldiers. He wonders why more humans are affected than Akwabi. Donna suggests that it is the environmental protection pills are the source of the infestation. Someone is duping the army. Nash endures a boring tour of the hospital before insisting that he see Lanova. He is surprised by the weight she has put on. There is an odd smell in the room. He tells her that he is not merely an embedded journalist but also the brother of a soldier (his sister) who was injured and treated at the hospital. Nash wonders what is so special that seriously injured soldiers can be returned to the front line. Two figures enter, one an intern, the other is Nash’s sister. She transforms in front of him into an insect. Lanova merely smiles.

The transport returns to the hospital the Doctor and Donna jump out. The Doctor wonders why the hospital is so far from the front line. General Brudge approaches but ignores the Doctor. The two Akwabi are removed from the truck. Padova is forced to kneel. The general slaps him across the face. The Doctor intervenes demanding that the prisoner not be mistreated and follows this up by implying that the general is a coward. Brudge demands to know what wars the Doctor has fought, prompting a comment from Donna to not get him started. A scream from nearby sets the Doctor running, followed by Donna. Apart from the sickly smell Donna first notices Nash in the grip of six foot insect while another stands nearby. Lanova watches. As a distraction Donna insults Lanova’s weight but this does not work as the insect snaps Nash’s neck.

The second Serfian immediately attacks the first. Brudge bursts in and pulls h is handgun, shooting the troops to the floor. Lanova inspects the bodies with interest. The Doctor accuses her of knowing about the insects all along. She agrees that she found some dormant eggs in a battle crater and has refined them into the soldiers’ inoculations to create the troops Brudge demanded. Brudge pulls his gun but the Doctor takes it from him and aims at the general who merely tells him that it is keyed to only his own palm print. The Doctor pulls the trigger three times to disarm the weapon. The general tells the Doctor that he doesn’t have the courage to bear arms.

Furious, the Doctor says he does not want to bear arms. Then, turning to Lanova, he pulls the table away from in front of the surgeon. Her lower half is a slimy, slug-like creature that extends into a hole in the wall into which she lays her Serfian eggs. She admits that she is breeding other queens, using former soldiers as her drones. The Doctor warns her that a Sharback robot is on its way. Lanova says, ‘Let it come.’ Brudge demands that someone give him a weapon to destroy Lanova. None of the nearby soldiers responds. All of them, including Merriam, transform into insects and surround the general. The push him to Lanova, who says that she is feeling hungry. The insects stop the Doctor from getting to the general before he is eaten. There is an explosion, the roof collapses and Donna is knocked out.

Shells are pouring into the camp, crumpling the hospital. The Doctor tries to get back into the burning building but Yovich will not let him in. The Doctor tells him that Brudge is dead and that Yovich is now in charge. Yovich assumes control. Firefighters begin to put out the blaze but Akwabi warriors arrive in the middle of the camp. There is a brutal exchange of fire. Just as the Akwabi seem to have won they halt. Padova canters towards him, handcuffed. On his back is Yovich, with his gun at the centaur’s temple. Then a dark silhouette is cast over them. The Sharback robot advances. All the soldiers turn to face it. The Doctor uses his screwdriver to free Padova and asks both of the leaders to help him stop the robot. Yovich says that his men will destroy it but Padova tells him how it wrecked the Akwabi headquarters. This confession impresses Yovich. The Doctor says he needs the hospital protected to see if he can use the facilities to overcome the effects of the inoculations. He says that soldiers that turn into insects become the target of the robot. Both commanders order their men to cease fire. Padova lifts the Doctor onto his back and they race over to the robot. Unfortunately the screwdriver has no effect on the robot’s progress, but it stumbles when the ground collapses beneath it.

A chasm opens and the robot falls into it. Padova cannot stop and crashes into the deep hole with the Doctor and Yovich. The three of them gaze into the gloom at an extraordinary structure of hexagonal cells stretching into the distance. In the cells there are thousands of Serfian drones. There is also a row of larger cells containing young queens. The Doctor observes that once the queens begin to lay eggs they will over-run the planet in less than a week.

The robot shudders, its head almost backwards. One of its limbs is severed. The Doctor unseals the rear panel of the robot and sees that the self destruct mechanism has been activated. Yovich begins to help the Akwabi to drag himself up the slope to the surface. At the top humans and Akwabi pull them out. The trio tell the soldiers to run. As they do so, a massive explosion erupts from the pit, shaking the ground and raining soil into the trembling hospital. A few Serfians fly out of the hole. The Doctor tells Padova and Yovich that they need to work together to hunt down the insects.

Donna wakes up face down o n the carpet. There is the smell of burning: Lanova’s rear end is smoldering. She is angry at the deaths of so many of her children but says that the death of the robot means that they will soon be replaced. She taunts Donna, saying that a Serfian is growing inside her. The Doctor bursts in. The Serfian guards grapple him to a halt. Lanova brandishes a hypodermic, threatening to inject him with the inoculation. The Doctor quotes the Hippocratic at her, but Lanova is unrepentant and taunts Donna about her weakness on the battlefield. Donna replies that there is more to bravery than killing and says that she would have rescued Merriam even if she had known that the young soldier was to turn into one of the insects that now holds her prisoner. When Lanova tells the Merriam-insect to take Donna away it turns instead on the surgeon and bites her head off. Lanova’s fingers tighten in a spasm on the trigger of her pistol and a shot kills Merriam. The other insects begin to stagger, leaderless. Donna covers the body of her former friend.