The TARDIS materialises in the Unulkahai system in the year 2731. Humans have colonised two moons of the gas giant Indra, terraforming Raghi and building a power facility atop an ancient alien structure on the desolate Agni, which orbits within Indra’s magnetosphere. Seeking a safe haven in which to overhaul the TARDIS, the Doctor rents a private villa on Raghi, assuring the worried Turlough that although they are indeed on the outskirts of the Tzun Confederacy, the Tzun themselves were wiped out by the Veltrochni over 200 years ago. Raghi appears to be quiet and peaceful at first, but while shopping for spare parts in the marketplace, the Doctor encounters a distraught woman who is seeking a doctor for her husband Chattar. The Doctor agrees to see to him, and finds that although Chattar seems to be suffering from nothing more than a bout of flu, his skin texture has altered, becoming coarser and harder. Paramedics from the Kshatriya caste arrive and take Chattar to hospital, but when they mention in passing that there have been other cases of this illness, the Doctor becomes concerned and demands to see the colony’s Preceptor.
Preceptor Ambika’s daughter Nur attends the opening of a new hospital, but finds its chief medical officer Jahangir to be oddly distant, and is surprised to see an odd ambulance with blacked-out windows rushing to the hospital as she leaves. Ambika is investigating the kidnapping of a noted pharmaceutical scientist when the Doctor and Turlough arrive, but much to Nur’s surprise, her father curtly dismisses the Doctor’s concerns that the colony may be experiencing an outbreak of a new disease. Nur decides to investigate on her own, and when she discovers that a number of chemists and biologists have disappeared in recent months she comes to suspect that her father is covering up the existence of a plague in order to prevent panic. Nur takes her concerns to the Doctor and Turlough, and tells them that the first reported case of the new symptoms occurred soon after an apparent meteorite shower caused a temporary loss of contact with the base on Agni. The situation was soon resolved with the help of the ship Nandi -- and Jahangir was its medical officer at the time.
The Doctor asks Nur, a qualified pilot, to take him to Agni to investigate, while Turlough remains on Raghi to determine whether there is any other connection between the missing scientists. Nur and the Doctor depart in Nur’s private flitter, the Garuda, but as they approach Agni, the Nandi unexpectedly appears and opens fire on them without warning. As Nur tries to escape, an alien ship decloaks in front of her and launches fighters, and she is forced into the atmosphere of Indra to shake them off. The alien fighters are designed to function only in the vacuum of space, and they succumb to atmospheric pressure and are destroyed; the Garuda survives, but is too badly damaged to return to Raghi safely. The Doctor, however, is able to recircuit its radiation shields to block sensor scans, and Nur lands on Agni undetected. She and the Doctor enter the base, where they discover that the alleged “meteorite shower” was in fact a successful attack and invasion by the Sontarans. The staff of the base and their families have been conditioned to serve the Sontarans, as have the crew of the Nandi, and they are all being worked to death by their new masters -- who have been kidnapping scientists from Raghi to replace those who have died from exhaustion.
During the dogfight with the Garuda, a glancing blow from a Sontaran fighter caused a minor explosion on the Nandi’s bridge -- and the shock has broken Captain Sharma’s conditioning. He manages to bluff the Sontarans into sending him to Agni for material with which to repair the damage, but upon arriving he fights off his guard, intending to escape back to the osmic projector which he can use to return to Raghi. The Doctor and Nur encounter them fighting, and the Doctor bounces a cricket ball off the Sontaran’s probic vent, stunning him. He, Nur and Sharma retreat to the base’s sickbay, which the Doctor assumes will be going unused -- but while pondering his next move, he finds that the Sontarans are using the bay to store genetic samples. Although untrained in medical matters, Nur recognises the pattern of the DNA tags -- she saw similiar genetic material while touring Jahangir’s new hospital. The tags are being attached to strains of the common cold virus, but such tags are merely used for identification purposes, and are not inherently fatal. So why are the Sontarans apparently attempting to tag the population of Raghi?
Turlough visits the spaceport on Raghi to find out if there is any connection between the missing scientists and Agni, and while enlisting the help of a traffic co-ordinator named Karan, he notices a strangely-shaped asteroid which Karan has been keeping an eye on. Feeling that the asteroid’s configuration is too regular to be natural, Turlough convinces Karan to hack into the military satellite net for a closer look -- and they discover that the “asteroid” is in fact a mile-long alien spacecraft. They set off to warn Ambika, but Jahangir has detected their unauthorised intrusion and has alerted the Sontarans. Two Sontaran troopers ambush them and kill Karan, but they hesitate when Turlough identifies them as Sontarans; as the human race has not officially encountered Sontarans yet, this implies that Turlough is not human. Turlough escapes while the Sontarans are distracted, and while hiding, he sees a robot supervising the addition of some sort of chemical compound to the Sudra caste’s water reservoir. Meanwhile, the troopers report their encounter to Major Karne, the officer in charge of the Indra operation, and Karne diverts the Scavenger robot to scan the city for alien power sources. It soon locates the TARDIS, and Karne, recognising it for what it is, has it taken to the Sontaran mothership. He also receives word of intruders on Agni, and, suspecting that they may be Rutan spies, he orders that they be taken alive for interrogation.
As the Doctor, Nur and Sharma evade the pursuing Sontarans, the Doctor realises that the humans have constructed their base atop an old Tzun facility made from the rare metal terullian. He and his friends reach the osmic projector, which the Doctor uses to transport them to a second terminal in the new hospital on Raghi. The Doctor sets the second projector on a jamming frequency to prevent the Sontarans from pursuing them, and then tracks down Jahangir and uses a pulsing light to free his mind from the Sontaran conditioning. Once restored to his senses, Jahangir -- nearly mad with grief at what he has been made to do -- reveals that he convinced Ambika to let him add the polymerase tag to the colony’s water supply in order to genetically differentiate between the castes on Raghi. All those whose bodies have rejected the new DNA have ostensibly been placed in quarantine in Jahangir’s new hospital -- but unbeknownst to Ambika, Jahangir has in fact been killing them to prevent the tag from being discovered. The Doctor and allies, reunited with Turlough, go to the palace and inform the horrified Ambika of the truth; but how will they expel the Sontarans from the facility on Agni without risking an explosion which could immolate all of Raghi?
Major Karne and his second-in-command, Commander Loxx, receive word that the first Rutan probes will reach the Unulkahai system within two days; the mile-long ship must be ready for nucleosynthesis by then. They also learn that Field Marshal Stentor has taken personal command of the mission to Antares; while the Rutans are distracted by the Unulkahai strategem, the Sontarans will attack their Antares base, download the Rutan data core, and destroy the base from orbit. Loxx reminds Karne to report the capture of the TARDIS, and Stentor, delighted, orders Karne to capture the Doctor alive. As their preparations near completion, Karne summons Jahangir to Agni to prepare for the final stage, and Jahangir returns, pretending that he is still under the Sontarans’ control in order to find a way to destroy them.
The Sontarans use DNA scanners to locate the Doctor at the Preceptor’s palace, but the Kshatriya soldiers fight off the attack. While studying a scanner taken from a dead Sontaran, however, the Doctor discovers that the Sudra caste members on the Preceptor’s staff are registering as Sontarans -- and the Sudra caste is the only one to have been tagged so far. Putting this discovery aside for the moment, the Doctor, Nur, Turlough and Sharma lead a patrol back to Agni to rescue the Sontarans’ prisoners, but find that the Sontarans are already evacuating into the mile-long spaceship which Karan and Turlough spotted earlier -- and which the Doctor recognises as the gravitational nacelle of a damaged Tzun Stormblade.
While Turlough and Sharma set about freeing the Sontarans’ slaves, the Doctor hacks into the Sontarans’ computer net to find out what they’re up to, but he and Nur are captured by Loxx and are taken aboard the Sontaran mothership for interrogation. Karne isolates them, to interrogate them personally, but while his back is turned, Nur hits him in the probic vent as the Doctor had shown her, and escapes -- unaware that he was only shamming unconsciousness. Karne then interrogates the Doctor, and reveals that the people of Raghi are being used as bait to lure the Rutan into a trap. Probes from the Antares colony will soon reach this sector, and when their long-range scanners detect the polymerase DNA tags, the Rutan will conclude that Raghi is in fact an isolated Sontaran colony. Meanwhile, a suicide volunteer will pilot the Stormblade nacelle into Indra’s atmosphere, and when the Rutan attack, the volunteer will detonate the Stormblade’s graviton drive, catastrophically increasing Indra’s mass and turning the gas giant into a new star. The attack fleet will be destroyed, and the cautious Rutan will mark off this sector of space as too dangerous and avoid it -- thus giving the Sontarans unrestricted access to the deserted worlds of the former Tzun Confederacy, and all of their precious supplies of terullian.
Nur is reunited with Turlough and Sharma, and they and the Kshatriyas storm the Sontaran ship and rescue the Doctor. Fighting a rearguard action against the Sontarans, they escort the Doctor off the ship before he can object -- and as it detaches from Agni and departs from the Unulkahai system entirely, he reveals, too late to do any good, that the TARDIS was aboard. Meanwhile, Jahangir frees the crew of the Nandi from the Sontarans’ control, and when Sharma warns him of the danger posed by the nacelle, he orders the others to evacuate and sets the Nandi on a collision course with the nacelle, seeking to atone for his crimes. Loxx has volunteered for the suicide mission and has already piloted the nacelle into Indra’s atmosphere, but as soon as Jahangir is close enough he disengages the Nandi’s anti-matter containment field, triggering an explosion which destroys the nacelle before it is deep enough within the atmosphere to trigger nucleosynthesis.
The Doctor, Nur, Sharma and Turlough then travel to the outskirts of the system and broadcast a message to the Rutan Host, asking to parlay. When the suspicious Rutan respond, the Doctor explains the truth and offers to trade the strategic data he downloaded from the Sontarans’ computer net, in exchange for the Rutan’s promise to leave the human colonists alone and to escort the Doctor to Antares so he can recover property which the Sontarans have stolen from him. The Rutan, realising that the Doctor is referring to a TARDIS, agree to his terms, but the Doctor is well aware that they’ve done so in order to steal the TARDIS themselves.
The Sontarans attack Antares, expecting minimal resistance -- but thanks to the Doctor’s warning, the Rutan have laid an ambush, and a fierce space battle breaks out. Using transponder codes which the Doctor downloaded from the files on Agni, Nur manages to fly the Garuda through the heart of the battle and dock with the Sontaran flagship, and while their Rutan defender fights off the Sontaran troops, the Doctor, Turlough, Nur and Sharma escape from the docking bay, locate the TARDIS and flee, leaving the Sontarans and Rutans to their war. Back on Raghi, Ambika chooses to resign his position and spend the rest of his life as a Renouncer to atone for his foolishness.
Meanwhile, back over Antares Three, the Sontarans attempt to carry out their original mission, and successfully download the data core from the Rutan base -- and in the process, they discover something which could change the outcome of the war. Desperate to stop the news from getting out, Major Karne breaks cover and reveals himself to be a Rutan spy, but although he manages to kill Field Marshal Stentor he is unable to prevent the Sontaran intelligence officers from broadcasting the data to every other ship within range. One ship escapes from the battle, taking the vital secret back to the Sontaran homeworld. Karne tries to escape in the Garuda to warn the Rutan of the danger, but the ship is punctured by shrapnel as it flies through the heart of the battle -- and once exposed to space, Karne freezes into immobility, doomed to drift until he is found and woken by some unfortunate passer-by...
Source: Cameron Dixon