This is the future; the Orion war continues with no end in sight, while the ion storm Protyon drifts to the fringes of the Garazone system. There, Captain Thinnes of the cargo loader Silver Jackal tricks a space patrol ship into leaving him and his associate Digly alone in a sector rich with unclaimed salvage. The Jackal soon locates the find of the century, an abandoned star destroyer which appears in perfect condition from the outside. Thinnes and Digly spacewalk over to check out their discovery, and a quick perusal suggests that it's in near-mint condition, apart from a foul stench in the atmosphere. But before they can conduct a more thorough investigation, something lurches out of the shadows and kills them both...

The Doctor has taken Charley to the bazaar on Garazone Central, one of the earliest artificial space habitats built by humanity. In this era, it's far from the central authorities, and one can find almost anything here -- perhaps even a text which might explain what's wrong with Ramsay. When one of the bazaar's inhabitants makes a rather unwholesome suggestion to Charley, the Doctor takes the offended girl into a shop and bluffs the shopkeeper, Ike, into believing that they are Customs and Excise officials so they can browse in peace. He's pretty sure Ike won't push the matter; smuggling is rife in this sector, and the Doctor saw Ike leaving a merchant space corps ship earlier. However, Ike has been nervous ever since Thinnes and Digly disappeared, and he thus contacts his associate Grash to warn him that they've been rumbled. Grash dismisses his fears, and tells him that there's a bigger problem; Captain Obermann has been transferred off the Vanguard, and the new captain, Deeva Jansen, has brought forward the ship's disembarkation time.

Ike bolts from the shop without closing up, surprising the Doctor and worrying Charley. They know he's a smuggler, and he believes that they're government officials; perhaps he's gone to fetch some of his friends to give them a seeing-to. Or, worse, as the Doctor told Ike where he'd landed, perhaps they're going to loot or steal the TARDIS. The Doctor, realising that Charley has a point, picks up a book of ancient remedies for Ramsay, and sets off to hire a grav-pad and beat Ike back to the docking bays. The grav-pad shorts out mid-flight, dropping them unceremoniously at their destination, but even so they're too late; the Vanguard's new captain is in such a rush to leave that she's ordered her crewman Vol to load the supplies aboard without double-checking them. And thus the TARDIS is accidentally loaded aboard the Vanguard as well. The Doctor and Charley have less than ten minutes to find a way onto the ship before it leaves, taking the TARDIS with it...

Ike arrives with seconds to spare, and joins Vol and Captain Jansen on the bridge as the ship blasts out of spacedock. Jansen then returns to her cabin, while Vol and Ike settle in for the flight and wonder what's up with their new captain. Why the rush to leave? Why is she packing such a heavy-duty firearm? Jansen seems too bright and ambitious for this dirty job; perhaps she's been demoted after having a row with someone on the space corps executive. Whatever the case, she's not your average scrapship captain...

Thanks to the Doctor's sonic screwdriver, he and Charley get into the Vanguard's waste bays moments before the ship leaves Garazone Central. As they search for the cargo hold containing the TARDIS, the Doctor explains to Charley that this ship is rather like an interplanetary dustcart, roaming the areas between the distant space lanes and picking up the rubbish collecting there. Charley is offended by the poorly maintained service areas, but the Doctor is more puzzled by the fact that he can sense the vibrations of a interstellar drive being pushed to its limits. Where are they going in such a hurry? Just as they find the TARDIS, the engines suddenly cut out -- quite a dangerous manoeuvre. The Doctor decides to get off this ship as quickly as possible, before the crew fly it to pieces.

Vol and Ike have cut power to the engines to prevent a collision with a massive, abandoned star destroyer, but the Vanguard was going too quickly when the proximity alarms went off, and even full reverse thrust isn't enough to stop their ship in time. Jansen orders them to re-initiate the hyperdrive and set it for a three-nanosecond reverse burst -- another dangerous manoeuvre, but one which saves them. It doesn't do much good for the TARDIS, however. The Doctor has patched his own ship's scanner into the Vanguard's systems and spotted the star destroyer outside; realising that the ships are about to collide, he attempts to dematerialise, just as Deeva fires up the warp engines. The TARDIS is caught in the warp field, causing a surge of temporal feedback in its time core. The Doctor materialises immediately, and he and Charley are forced to abandon the TARDIS before the temporal feedback lashes out into the console room. Ramsay should be safe enough -- the Time Vortex is his natural habitat, and the release of temporal energy might even be good for him -- but the Doctor and Charley will be stuck outside the TARDIS until the energy has dissipated. As it happens, the TARDIS has materialised on board the star destroyer, and the Doctor decides to explore...

Deeva runs into a problem while trying to get her recon team over to the destroyer; she's met her match in the team leader, Grash. Standard operating procedure is for the scrapship to link to the abandoned vessel with a docking tube, but the destroyer's airlock is too large, and Grash refuses to spacewalk; it's not in his contract. Deeva is forced to resort to offering them all double bonuses, and the team members -- Grash, Chev, and Kelsey -- suit up and cross over to the destroyer. Deeva returns to the bridge, where she finds Ike hacking into her command terminals to learn more about the star destroyer. It's been spotted before, but no other scrapship was willing to take on a job of this size; but that's all he learns before a top-priority security block closes down the data. The only way for him to learn more about the destroyer is through the ship's secure terminal, and only Deeva has access to that.

The Doctor and Charley hear screaming and rush off to investigate, to find Kelsey lying before the TARDIS, battered nearly to death. As they try to stop the bleeding, Chev and Grash arrive, see them with the body and leap to the wrong conclusion. Kelsey dies while the Doctor is busy trying to convince Grash that they aren't responsible, and Grash contacts Jansen and requests authority to execute the Doctor and Charley on the spot. However, she realises that they can't be responsible for the brutal violence of Kelsey's death, and instead she orders Ike to take two spare suits over to the destroyer and bring the strangers back to the Vanguard. Ike does so, painfully aware that while Jansen is monitoring him he can't tell Grash that the Doctor and Charley are the strangers from the shop...

Back aboard the Vanguard, Ike and Chev take Kelsey's body to the sickbay while Jansen and Grash question the Doctor and Charley. Grash accuses them of being rogue androids from the Orion sector, but Jansen knows he's just grasping to explain the damage done to Kelsey's body. Vol then calls from the flight deck, informing them that he's picked up a transmission from the airlock reception area. Grash confiscates the Doctor's temporal tracker and sonic screwdriver, but the transmission continues, and its source begins to move. The Doctor spots movement in the darkness, but whatever it was has gone when he looks again; however, it has left scratches on the deck plates. Moments later, the lights fail in the reception area, and faults begin to spread throughout the ship, from the sickbay to the flight deck. Despite Grash's reservations, Deeva shows the Doctor to the nearest maintenance hatch, where they see that something has burned through it and the power cables beneath. And yet the life-support cables are untouched, suggesting that the intruder doesn't want the crew dead...

As the disruptions spread, it becomes obvious even to Grash that the Doctor and Charley can't be responsible. He and Chev remain suspicious, but Ike and Vol at least concede that Kelsey's wounds would have left his attackers drenched in blood. The Doctor has a theory to explain one of the mysteries, and when he examines the team's backpacks he finds scratches on Grash's. The intruder must have stuck to Grash's backpack on the destroyer, and dropped off in the airlock while he was concentrating on the Doctor and Charley. The Doctor suggests shutting down life support in the empty areas of the ship and diverting the spare power to the security systems to locate the intruder. Charley remains on the flight deck with Deeva and Grash, while Chev and Vol supervise the Doctor. The Doctor completes his work, and Vol locates the intruder; it's reached the stern of the ship, and is still transmitting the same signal he detected earlier.

Grash just wants to pack up and go, but Deeva has no intention of abandoning what could be the biggest haul in salvage history. Grash thus accuses her of unnecessarily endangering her crew, places her under arrest, and orders Vol and Ike to take the ship back to Garazone Central. But when they try to obey, they find that the intruder has reached the engines; the Vanguard isn't going anywhere now. Grash has no choice but to let Deeva and the Doctor cross over to the destroyer as they wish -- but Charley must remain here as a hostage, and if Vol gets the engines working, Grash isn't going to wait for them to return. Once the Doctor and Deeva have gone, Ike finally gets the chance to tell Grash about his earlier encounter with the Doctor and Charley; but if they really are from Customs and Excise why haven't they reported Ike to the captain? Grash doesn't know, and he doesn't care; Charley poses a threat, and he knows how to deal with it and solve their engine problem at the same time. Ike and the reluctant Vol thus force Charley to accompany them to the engine room, where she realises that they intend to use her as bait to lure the intruder out into the open -- and get rid of her in the process...

The Doctor suspects that there's more to Deeva, and her interest in the destroyer, than she's letting on. But another mystery gets answered first, for moments after he and Deeva board the destroyer, they are attacked by Kelsey's killer -- a rogue Cyberman. To the Doctor's surprise, one blast from Deeva's gun is enough to destroy it, and she knows what it is even though the Cybermen have been entombed on Telos for some time. She claims to have studied that period in military history... which again raises the question of what such a well-educated and trained officer is doing in charge of a scrapship. The Doctor studies the dead Cyberman, and finds that its cyber-neural systems were damaged by faults caused when it was prematurely woken from cryogenic stasis. The Doctor now suspects that the thing on the Vanguard is a Cybermat -- and as the signal from the Cybermat wasn't meant for this rogue, that implies that there are other Cybermen aboard. The Doctor and Deeva must find them and prevent them from waking...

While some of the revived Cybermen attempt to overcome the power failures plaguing their ship and revive their Leader, two warriors cross over to the human ship to begin the next phase of the operation...

Ike forces Charley into the engine room, where she finds the Cybermat waiting for her. Grash orders Vol and Ike to wait until it kills her before destroying it, or else kill her themselves and claim that it did. However, Vol can't bring himself to do that, and he shoots the Cybermat first, putting it out of action and saving Charley's life. However, when he and Ike enter the engine room they find that the engines themselves are beyond repair. Meanwhile, Chev and Grash detect someone at the airlock, and, assuming that the Doctor and Deeva have returned, go to arrest them again. But when the airlock opens two Cybermen enter, and Grash and Chev's guns have no effect on them. Chev flees as the Cybermen shoot Grash with a stun-bolt and condition him to obey their commands.

Deeva's tracker locates a large chamber worth investigating, but the door has been fused by a power surge -- perhaps the same one which caused the rogue's hibernation unit to malfunction. Deeva suggests that the surge may have been caused by one of the ion storms common to the sector, and when she and the Doctor force the door open they find evidence to support this theory -- a conversion chamber filled with corpses. This is a factory ship, sent out from Telos to kidnap humanoids and convert them into Cybermen -- but when the storm hit the conversions were halted midway, and these half-converted prisoners died in agony. Beyond the conversion chamber lie the cryogenic units, where the Doctor and Deeva see the Cyber Leader finally waking. The other Cybermen inform their Leader of the systems failures caused by the ion storm, and of the fact that a Cybermat has sent the reactivation signal; a human ship is alongside, and its crew are to be converted. The Doctor and Deeva have heard enough, and the Doctor decides to retreat to the TARDIS while he ponders what he's seen -- and why the Cybermen seem to be reviving their warriors one at a time. However, the Cybermen reactivate their ship's security systems and detect the presence of intruders, and the Doctor and Deeva are forced to retreat to the airlock. For some reason, the Cybermen don't follow them out. The Doctor wonders why, not realising that two warriors are already aboard the Vanguard, waiting for them to arrive...

As Charley and Vol study the Cybermat it comes back to life, but without its weaponry it seems harmless. The Vanguard's engines, however, are a total write-off. Chev arrives, terrified and believing that Grash is dead, but as she tells the others about the invaders Grash arrives; he seems somewhat out of sorts, but claims to have killed the invaders. However, he also claims that there are more on their way, and insists that the others help to drive them off. They reluctantly return to the airlock with him, to find that the Doctor and Deeva have just entered -- and that Deeva has destroyed the waiting Cybermen. Grash collapses when he sees the warriors lying dead, and the others decide that the mutiny is now over; they'll all have to work together to escape the Cybermen. While Ike and Chev carry Grash back to his cabin, Vol jams the servo-mechanisms of the airlock, and then takes the Doctor and Charley to the flight deck. Deeva will join them later; she has other business to take care of first. Meanwhile, the Cybermen prepare to take the Vanguard by force. Contact is re-established with the Cybermat; it has lost its offensive capability, but it has other programming...

Vol warns the Doctor and Charley that if Grash thinks they know about Ike's sideline, they're in bigger trouble than they can imagine. They might claim not to care about petty criminal activities, but there's nothing petty about Grash... Ike and Chev then arrive, and the Doctor questions them about Deeva; he wants to know why someone like her is captain of a scrapship, and how she got hold of a gun which can destroy a Cyberman with virtually a single shot. Ike tells the Doctor about the classified file he was unable to access, but before the Doctor can investigate, an alarm sounds; the Cybermen are trying to break in. Ike calls Deeva to warn her, interrupting her as she uses the secure computer server to look up information on the "Doctor" and his "TARDIS". Apparently her mysterious visitor is an expert on Cybermen...

Deeva joins the others at the airlock, where the others suggest letting them in so Deeva can blast them. However, she can only take down one at a time, and if she misses just one, there will be no hope for any of them. The Doctor has an alternate plan, albeit a dangerous one; as soon as the Cybermen enter the airlock, he jettisons the outer door. Since the Cybermen want the humans alive for conversion, and they can no longer open the inner door without evacuating all of the air from the Vanguard, they are forced to remain where they are. The Doctor has bought some time, but not as much as he thought; the Cyber-Leader re-establishes contact with Grash and the Cybermat, and puts its backup plan into motion. By closing the bulkheads around the reception area, Grash can create a secondary airlock. The Cybermat will assist him; its weaponry may have been destroyed, but it is still capable of bringing any further humans it encounters under Cyber-control...

Leaving Vol on watch, the others retreat to the flight deck to consider their next move, while Chev checks on Grash. However, in this moment of calm it occurs to Charley to wonder why Grash passed out earlier, and Ike and Vol realise belatedly that if Deeva hadn't returned when she did, then Grash would have led them straight to the waiting Cybermen. The Doctor realises too late that Grash was under Cyber-control; when Chev arrives at Grash's cabin, he's gone, and he's taken his gun with him. Deeva orders Chev to follow him to airlock reception and sets off to join her there -- and the Doctor decides to take the opportunity to access the classified file which Ike failed to get into earlier. He manages to do so, and discovers that it's a Garazone space patrol report; the Silver Jackal was found abandoned near these co-ordinates, and the presence of the star destroyer was reported to military intelligence. All further data is locked under Tellurian security classification Orion D-7, and the computer shuts itself down when the Doctor tries to access it anyway. Charley recalls that when Grash accused her and the Doctor of being androids, Deeva claimed that there were no androids outside Orion. It seems the time has come to learn why.

The Orion war; to Ike it's current events, to the Doctor it's past history he needs reminding of, and to Charley it's an appalling glimpse of her race's future. Humanity has been building androids for some time, and they've finally made them advanced enough to think of themselves as real and start demanding rights. When the authorities refused, the androids withdrew to the Orion sector, where most of them had been made, and ordered the humans there either to accept them as equals or get out. War was declared on the "rebel" androids, and it's been going on for eight years now, with no end in sight. The Doctor still doesn't know how this connects to events here, and he's too concerned with the immediate problem to pay much attention to a brief movement he spots on the main viewport, off in the distance. Whatever it is has gone when he looks again, and as Ike doesn't have the power for a long-range scan, the Doctor dismisses it as unimportant and sets off to break into the secure computer module...

Deeva contacts Vol, who informs her that the Cybermen are attaching a control device to the inner door. Realising what they intend to do, Deeva orders him to get out of there and join her. He doesn't need telling twice, and as they shelter outside the reception area Deeva calls Ike and tells him to send the Doctor down. Ike is forced to tell her that the Doctor isn't there, and, realising what he must be doing, Deeva sets off to stop him, leaving Vol alone. As he waits, Chev arrives and tells him that she's found something for him to look at. He steps forward -- to find the Cybermat waiting for him...

The Doctor and Charley break into the secure computer module, which recognises the Doctor from Deeva's earlier research. The Doctor programmes it to believe that since it knows who he is, he must have security clearance, and although hesitant at first it eventually starts to tell him all it knows about Orion classification D-7, the "Sword of Orion". The destroyer has been spotted by several ships in this area, and military intelligence has determined its nature; as the Doctor had suspected, "Deeva Jansen" is not a merchant corps captain at all, but an Earth security operative on a mission to locate the star destroyer and determine the military value of the Cybermen. Deeva arrives and switches off the system before the Doctor can learn more, but he's heard enough to utterly disgust him; if Deeva thinks he's going to help her recruit the Cybermen to help win the war in Orion, she's very much mistaken. However, it appears that she has something else in mind...

Ike contacts Deeva to warn her that Vol, Chev and Grash are letting the Cybermen into the ship through their secondary airlock. Furthermore, he too has spotted something in the distance of deep space. The Doctor, Deeva and Charley return to the flight deck, arriving just in time to save Ike from the first of the Cyber-scouts, but there are more coming and there's no other way out of the ship. Deeva orders the others to don their suits, and shoots out the main viewport, intending to spacewalk over to the destroyer and try to escape in the Doctor's TARDIS. For some reason, the Cybermen don't follow the fugitives; it's almost as if they can't find them. Which is exactly the case, as the Doctor realises when he finally gets a good look at what he and Ike spotted through the viewport earlier. There's an ion storm on its way, and even at this distance it's making a mess of the Cybermen's scanners.

The Cybermen also detect the approach of the ion storm, and realise that they must return to hibernation at once. In the meantime they will convert the humans they have already captured. Vol, Chev and Grash are taken to the conversion chambers and put in place, and the control over their minds is relinquished while the conversion process begins. It's terribly painful, and as they enter the secondary stage Vol and Chev give in to the process... but Grash simply keeps raving. He's the criminal kingpin of the Garazone system, and nobody does this to him and his people; nobody! The Cybermen, realising that Grash has gone insane and is no longer suitable for conversion, terminate him.

The survivors' oxygen supplies are limited, and they are forced to board the destroyer through the airlock despite the risk that the Cybermen may be waiting for them. As indeed they are. To the Doctor's great disappointment, Deeva identifies him to the Cybermen and claims to have a proposition from the Earth Alliance. Despite the Leader's conviction that logic will find a way out of any situation, they can't deny that it will be difficult to return to Telos and revive their race in a ship devastated by ion storm damage. Deeva claims to have the authority to repair this ship in exchange for the Cybermen's help in fighting the androids; the Cybermen would become the Sword of Orion. Deeva is surprised by Charley's disgusted reaction, but the Doctor and Charley insist that they don't simply support the androids over the humans; it's not that facile a question of morality. If humans made androids as intelligent beings, then it's humanity's responsibility to face up to the consequences.

The Leader dismisses both the moral discussion and Deeva's proposition, and orders that the Doctor be frozen for analysis on Telos while the humans are sent to the conversion chamber. But the Doctor offers a counter-proposal, and backs it up with logic. If the Cybermen's main drive was active they'd be moving now, to get out of the way of the ion storm; if their weapons were active they'd have taken a shot at the Vanguard earlier to prevent it from leaving; and if their revival systems were functioning properly they wouldn't have to revive their warriors one at a time. The previous storm did more damage than they're willing to admit, and the next will likely tear the ship apart... unless they agree to release their captives, in which case the Doctor offers to use his TARDIS to rig up a defensive shield. The ship is struck by the outer fringes of the ion storm, and the Leader reluctantly accepts the Doctor's proposal -- but rather than wait for the Cybermen to betray them, Deeva immediately opens fire on her guards. The Doctor, Charley and Deeva escape, but Ike is recaptured and sent to the conversion chamber.

Deeva is hit by a glancing blast while fleeing, and as they near the TARDIS the Doctor insists upon seeing to her injury. She resists at first, and the Doctor realises why when he sees fibre-optic circuitry sticking out of the wound. Deeva is forced to reveal her final secrets; she's an android double-agent sent to infiltrate this mission. The Earth Alliance never intended to make a deal with the Cybermen at all; Deeva made that up to buy time. In fact, the Alliance intended to learn the secret of Cyber-conversion, and use it to transform their own men into cybernetic troops with which to win the war. Deeva's real mission was to bring that information to Orion, where the androids would convert prisoners of war into cybernetic troops for their own side. The main body of the storm is now starting to hit, and the TARDIS is their only way out -- but the Doctor refuses to open the door until Deeva agrees to his terms. He will take her home, but only if she agrees to erase the information she has stored in her memory about the conversion process. He's aware that she can use force and threats against him and Charley -- everything a real human is capable of -- but he won't be moved. Perhaps it's the casual way in which he refers to Deeva's "people", or the fact that he admits he can only trust her to keep her word, which wins her over. She informs him that she has erased the data from her memory -- but they've left it too late. The Cybermen catch up to them, and the Leader seizes Deeva's gun and destroys it before she can bring it to bear. One of his warriors grabs Charley, threatening to dismember her if the Doctor doesn't let them into the TARDIS -- but then the ion storm strikes in earnest, breaching the hull. In the confusion, Charley finds that she can't breathe; her suit is secure but the Cyberman which was holding her has crushed her life support pack. As the ion disruption becomes too much for the Doctor to handle, he passes out, and the last thing he hears is Charley struggling with Deeva...