The Earth Alliance detects an anomaly approaching from the fringes of the Seriphia galaxy, and the starship Victorious is sent to investigate. Captain Medem discovers that a fleet of alien ships is hurtling towards Earth's galaxy and is refusing to respond to any hails. Even as he reports to Admiral Cheviet of the Rapid Response Fleet, the alien fleet drops its defense shields, accelerates to an unbelievable velocity and opens fire on the Victorious, which does not live up to its name. The invasion of the Daleks has begun...

Alby Brook is, or appears to be, an ordinary water-taxi driver in the marshlands of Vega 6, and Susan Mendes is his favourite customer. Her employers at the Rhinesberg Institute have sent her in search of rare mineral deposits, but she's more than willing to take a break from her work to open up a picnic with Alby. And, although she tries to hide it, she's disappointed to learn that he's thinking of quitting his job and taking a position on a cruise liner. But then the future comes in the shape of a burning starship, plummeting through the atmosphere to crash in the south sector of Vega City. As Alby and Suz rush back to the city, they find that they are only able to pick up local broadcasts; the satellites have been taken out. Ed Byers of planetary control must admit to interviewer Gordon Pellan that the Earth Alliance has engaged an unknown enemy just outside the Vega system -- and just as he announces that all contact with the fleet has been lost, a Dalek breaks in on the transmission, announcing that the Vega system is now a part of the Dalek Empire.

Alby, horrified, urges the bewildered Suz to leave the planet with him before the Daleks land and begin exterminating the population. But Suz only knows the Daleks from obscure history lessons, and is appalled when Alby cold-bloodedly tells her that her family might as well be dead if they haven't evacuated yet. Furious, she storms off to find them, leaving Alby to save himself... and, to his shame, he does. The Aquitania is the last ship to escape before the Daleks swarm in on Vega 6, laying cities to waste, bombarding the planet, scorching the land, leaving the few survivors to choke in radioactive dust plains. The weak and wounded are exterminated, Robotising plants are put into operation, and the Daleks begin drilling through the crust of the planet. In the meantime, they keep an eye out for any slaves who rate highly on a certain scale of behaviour -- such as Suz. She's made a new acquaintance on the plains, an elderly man who has nearly given up all hope of survival; but as she tries to support him to the slave pens, a Dalek realises that he isn't walking on his own, and orders her to move away from him. She refuses, nearly broken by all the horror she's seen in the past few days, but for some reason the Dalek doesn't exterminate her; instead, it reports her defiance to the Dalek Supreme, and is ordered to let her live. Suz and her new companion are forced into the cells, unsure how they managed to survive but unwilling to press their luck any further...

Alby makes a new acquaintance at the Aquitania's bar; newsman Gordon Pellan, who also managed to get off Vega 6 at the last possible moment. Alby drunkenly tells Pellan all about how he left Suz behind; he was only doing his duty, but blames himself nevertheless, and is coping by getting as drunk as he can. He staggers back to his cabin to sleep it off, but is woken in the middle of a severe hangover by a call from his superior officer, Tanlee. Alby is in fact a corporal in the Earth Alliance, who was given a suspended sentence for petty theft and was sent to Vega 6 undercover in search of Kalendorf, a former Knight of Velyshaa. After six months, however, all he had were vague clues and rumours, and now the Dalek invasion has put paid to his mission, for the Daleks are well-known for exterminating indigent populations. Alby is to be reassigned, and he'd best forget about Kalendorf... and about Suz.

The Daleks finish their drilling and set their slaves to work. Suz and her new acquaintance are marched to the mine shaft along with the other survivors, and once again, her hostility is noted and monitored. The slaves are not permitted to use mining equipment for some reason, and the work soon takes its toll, as does the constant supervision by zombie-like Robomen. Failure to work, or any hint of insolence, is punishable by whipping or death. Only Suz knows what the ore they are mining really is -- veganite, a rare mineral which the Daleks somehow must have known was here. When her friend asks what's so important about it, she automatically tells him that's classified, and they both burst out laughing when they realise what she's said. The Daleks and Robomen, unable to understand the sound of laughter, force them back to work... and Suz's new friend quietly tells her his name at last. It's Kalendorf.

As Alby prepares to set off on his new assignment, Pellan intercepts him with potentially good news. Some of Pellan's contacts have chartered an Earth Alliance ship, and intend to return to the Vega system to broadcast anti-Dalek propaganda -- and they've been picking up transmissions which seem to prove that some of Vega 6's population is still alive. Alby is surprised, but duty calls, and he is forced to board the waiting scout ship and accept his new orders from its Drudger pilot. To his disgust, he's being sent to a planet on the other side of the galaxy -- but then three Dalek ships attack the Aquitania, and he barely manages to get away in time. It appears that the Daleks have made an unexpected advance in this sector, and as far as Alby's concerned, this means that he can't risk trying to contact the Earth Alliance again; his new mission will have to wait. The Drudger can find no fault with his logic, and reluctantly sets course back to the Vega system on Alby's orders.

Whatever makes a person go on when all hope has been lost -- the fear of death, the involuntary physical desire to cling to life -- whatever it is, the slaves on Vega 6 have lost even that. People are dropping by the hundreds, and even the Robomen are too fatigued to force the slaves back to work. The Daleks can shout and threaten as much as they want, but their slaves simply cannot work a moment longer. Angry and frustrated by the Daleks' inability to comprehend this, Suz lashes out at them again... and this time, the images of her defiance are broadcast to the Dalek Supreme in Seriphia. Veganite is too volatile to be mined by Dalek technology, and the quotas have not been achieved... and yet, strangely, the Dalek Supreme orders that the slaves are not to be executed. Instead, they are to be allowed to rest, and Suz is to be brought to a communications chamber to speak directly to the Dalek Supreme. The Emperor Dalek will monitor their discussion personally, and no other Daleks are to be permitted to observe. What they are about to attempt is terribly dangerous; the fragments of data obtained from the failed assault on the Kar-Charrat library have provided invaluable data on the human mind, but that knowledge must never be communicated to the rest of the Dalek race...

Suz is left alone in the communications chamber while the Dalek Supreme speaks to her from the Seriphia galaxy -- a galaxy which the Daleks created themselves, with the use of a force known as the Apocalypse Element. The Dalek is not telling her this to threaten her, however; it is simply setting the basis for their discussion, for it is important that she understand that there is no way the Daleks can be defeated. Given that their victory is inevitable, why does she defy them? It takes Suz a moment to realise that the Dalek Supreme is genuinely waiting for an answer, but she's too exhausted to think clearly. If the Daleks allow their slaves to rest, then the work will stop, and this is unacceptable; and even if they work their slaves to death, they can always bring in more, from other worlds. Suz cannot counter these arguments, and she can't explain why she stood up to the Daleks and tried to convince them to give her people food, rest, and hope. Frustrated and exhausted, she tells the Dalek supreme to just go ahead and kill her if that's what it wants... but instead, it informs her that the slaves will be allowed to rest. And then, Suz and the Dalek Supreme will speak again. The Emperor is satisfied; everything is going according to plan...

Despite his Drudger's objections, Alby returns to the Vega system, risking death or worse for the impossible hope of finding Suz. But the odds are against him, and he is soon attacked by twenty Dalek trans-solar discs. Despite his best efforts, the Daleks take out his scout ship's engines -- but as they close in for the kill, an Earth Alliance ship arrives and destroys them. To Alby's surprise and delight, the Earth ship is broadcasting Vega Free News bulletins, and when he docks with it, Gordon Pellan is waiting for him. Pellan claims to have escaped from the Aquitania when the security guards foolishly tried to resist the Dalek invaders -- but he is deeply shamed and scarred by the fact that he ran in blind panic, climbing over screaming people to get to safety himself, giving no thought to anything but his own survival.

Kalendorf is surprised when Suz returns in one piece, and even more so when the Daleks allow their slaves to eat and sleep. Suz admits that she doesn't trust the Daleks either -- although she's rather puzzled that Kalendorf, like Alby, also seems to know the Daleks better than might be expected. For whatever reason, however, they have a breathing space to live and think of a way to fight back. Meanwhile, the Daleks on Vega 6 continue to follow their orders, allowing the slaves to rest even though this seems contrary to their plans for the invasion of Earth's galaxy. The Earth Alliance fleet is retreating before the Dalek advance, but to maintain their advantage the Daleks must exploit the mineral wealth of all their conquered planets in order to build more ships. Slave labour must be exploited fully, to conserve the energy and resources needed for the war effort... and the Emperor has a plan to utilise this labour effectively. In the meantime, the Earth ship in the Vega system will be allowed to survive, and the slaves on Vega 6 will be allowed to rest, even though this causes the work to fall behind schedule.

Suz is summoned by the Dalek Supreme again, and goes in hope, despite Kalendorf's reminder that these are the creatures who exterminated nearly every living thing on Vega 6. Indeed, upon her arrival in the communications room, the Dalek Supreme reminds her that she told it to kill her the last time they spoke. It asks her if she still wants to die -- and again, it seems to be waiting for a genuine answer. When it mentions her discussions with Kalendorf, she realises that her every move is being monitored. The Dalek Supreme admits that veganite is too volatile for the Daleks to mine with automated machines; they thus require slave labour, and while they don't care whether their slaves live or die, they acknowledge the potential benefit of a system which will enable their labour force to work continuously without dying. The Daleks cannot work out such a system themselves, for they lack the Human Factor, that indefinable quality which differentiates human beings from Daleks. Suz must create such a system for them if she wants her people to have hope again; otherwise, as Suz had feared they would, the Daleks will simply work their slaves to death and bring in more from offworld.

Suz is returned to the mines to consider, and she warns Kalendorf that they are being watched. But the Daleks can't monitor their thoughts, and to Suz's surprise, Kalendorf reveals that he is telepathic; he learned this technique and others from the Knights of Velyshaa, and they thus have a moment in which to share their concerns before the Daleks separate them. Suz is ordered to address her people, and although she's terrified at first, Kalendorf is there to offer his support. Thus, Suz speaks to her fellow slaves, telling them just what the Dalek Supreme has told her. The Daleks don't care whether the slaves die, which means that they have a chance to live. If they work in eight-hour shifts, and give the Daleks what they want, then they will be allowed to live; and as long as they're alive, they will have hope for the future. Her people listen to her, desperate for some thread of hope to cling to... but for the rest of her life, she will wonder whether this was defiance, or collaboration. Was she standing up to the Daleks the only way she could, or was she caving in to their demands in order to survive?