These accounts of Susan's origins have not been reflected in any other source. The story states that Susan is not in fact related to the Doctor, but is instead a descendant of Rassilon. This is a retcon which goes against the intent of the original programme makers who devised the character of Susan as the Doctor's granddaughter. A different account of Susan's origins, in which she is not exactly the Doctor's granddaughter, but instead the granddaughter of the Other, who was later reincarnated as the Doctor, is found in the novel Lungbarrow.
The impetus to say that Susan was not actually the Doctor's descendant came from the belief, widely held among fans in the 1980s and 1990s, that the Doctor is asexual, and thus could not have any actual offspring. This belief was also the official policy of producer John Nathan-Turner, who was often quoted as saying, "There's no hanky-panky in the TARDIS." Thus, this retcon by script editor Eric Saward was in tune with the intentions of the 1980s production team, even if it was at odds with the intentions of the original 1960s production team.
In the 21st century revival of Doctor Who, the Doctor has been depicted as a sexual and occasionally flirtatious being, and he has referred to himself as having been a father (in TV: The Empty Child, Fear Her and The Doctor's Daughter), so the notion of him also being a grandfather is generally no longer considered an issue. In the 2013 episode TV: The Rings of Akhaten, the Doctor, for the first time since the revival began, explicitly states that he has a granddaughter.
Tegan is not happy when the Fifth Doctor decides to answer a distress call from Sector 19, in the spiral of Alpha 4, a remote area on the fringe of a supernova. Arriving near the source of the signal, the Doctor is disturbed to find it is coming from an enormous battle cruiser. The cruiser appears undamaged. Unlike other warships, it doesn't carry any identifying insignia or identification. The Doctor, Tegan and Turlough are even more alarmed when they find that the TARDIS is being drawn to the cruiser by a tractor beam that is somehow using the TARDIS' own power against it.
The Doctor, working under the console, shuts off the TARDIS's power for a moment. There is enough energy left to dematerialise when the break in power releases the TARDIS from the tractor beam. The TARDIS materialises safely onboard the battle cruiser. They explore the vast craft in near total darkness — bar the minuscule light from the Doctor's pen torch. The Doctor, Tegan and Turlough are blinded and disorientated by a bright light when a door opens. A moment later they are prisoners of three Cybermen who appear to be assisting the Master.
The Master reveals that the battle cruiser is his TARDIS, disguised as a battle cruiser (with his working chameleon circuit) to suit the Cybermen. The Master's plan is to bring the Time Lords to their knees, not by war but by "securing power by legally exploiting the constitution!" As they are marched through the TARDIS corridors, the Master talks of Gallifrey's past. He recounts the final corrupt days of Lord President Pundat the Third. The Master remembers the Doctor as a crusty, eccentric man, one of the finest TARDIS engineers ever.
The Master remembers more than the Doctor, who realises he has had selective memories wiped – just as the Master had told him. The Master explains that students revolted against Pundat the Third but were ruthlessly put down. The students wanted to replace Pundat's rule (in which the Lord President chose his own successor) with Rassilon's Law, hereditary rule by a descendant of Rassilon.
The students had tried to recruit the Doctor, but he he had decided to side with the status quo, the constitution and Lord Pundat. When Pundat died of stress soon after the revolt, his chosen successor was the evil Chancellor Slann. The students had found the last of Lord Rassilon's descendants: Lady Larn, a seven-year old child adopted by Councillor Brolin. They decided on a second coup. Yet in trying to convert the Doctor, the students were overheard. The Doctor, innocent of the students' revolt, was too highly respected to be terminated like the other students. It was decided to wipe parts of his memory.
Bloody reprisals against the students followed, and the Doctor decided to leave Gallifrey in a TARDIS. As it happened, the Lady Larn was hiding in the same TARDIS that the Doctor stole; the Doctor knew her as Susan and she affectionately called him "grandfather".
The Master, Tegan rightly guesses, was behind both student uprisings. Believing the students ready for the second, he assassinated Lord President Slann. However, the students weren't ready and the Master fled.
Once more, claims the Master, Gallifrey is leaderless and politically unstable, the people are crying out for leadership, for a descendant of Rassilon. The Master plans to return with his army of Cybermen (to whom he has promised the knowledge of time travel) and rule with Susan as his puppet. However, the Master realises that Doctor, his biggest adversary, must be disposed of first.
When Susan is reunited with the Doctor, she refuses to help the Master. She defends the Doctor from the Master and his Tissue Compression Eliminator. As the Cybermen move to separate the Doctor and Susan, the Doctor launches himself at the Master, knocking the Eliminator from his hand. Susan fires as the Cybermen close in. The Master tells the Cybermen not to shoot lest Susan be killed.
In the stalemate the Doctor leads his companions back to the TARDIS. With the Master's exposed roundels in the corridor draining the energy to maintain the vast battle cruiser, Susan stops and fires the Eliminator. The damage destablises the Master's TARDIS. The Doctor, Tegan, Turlough and Susan just make the safety of the Doctor's TARDIS. The Doctor sets the co-ordinates to return Susan home. Outside, the Master is consumed by hate as the walls around him collapse and his TARDIS disintegrates, but the Doctor believes that he hasn't seen the end of him.
- Tegan mentions the Daleks. The Doctor recalls the Ice Warriors and the Terileptils.
- Cyberman blood/fluid is green.
- The Doctor says "Brave heart, Tegan".
- The Doctor carries a pen torch in his pocket.
- The Master threatens the Doctor with his Tissue Compression Eliminator. Susan uses it to kill two Cybermen and damage the Master's TARDIS.
- The Doctor and the Master have a rivalry over their TARDIS models, along with chameleon circuit and the Master's TARDIS being a newer model.
- When the Doctor asks if Lady Larn escaped the student uprising, the Master replies that they allowed her to escape: "...Even you must know they do not kill children on Gallifrey."
- 'TARDIS' is spelled 'Tardis' throughout this story.
- 'Rassilon' is misspelled 'Rasillon' twice in this story.
- Why the Master thinks he can control Susan, whom he allows to run around freely and be reunited with the Doctor (thereby foiling his attempt to kill him), is not explained.
- Lady Larn is said to be seven years old when she left Gallifrey with the Doctor, but by the time of An Unearthly Child she is fifteen. This suggests the Doctor and his 'granddaughter' had been travelling together for eight years before arriving in Totter's Yard.
- In the illustration of Susan firing the Master's Tissue Compression Eliminator, the weapons does not resemble the TCE as seen in any TV story. Likewise, in the illustration of the Master shooting Lord President Slann, he is incorrectly depicted as being in his current incarnation.
- The Fifth Doctor is illustrated without a stick of celery on his lapel.
- The name of the character Larna, from The Infinity Doctors and Unnatural History, appears to be a reference to Susan being named as "the Lady Larn" in this story.
- The Doctor's reflection that, although he travels with companions he doesn't know them very well, seems to be echoed in the series, with more backstory and insight given to the Doctor's current and future companions. This was developed over the next few years (most notably with the Seventh Doctor and Ace's relationship) and became a central theme of the new series companions.
- This Doctor wishes he still had his sonic screwdriver. (TV: The Visitation)
- The Master's assassination of Lord President Slann predates his murder of the Lord President in TV: The Deadly Assassin, although the assassination of the Lord President in that story is strongly suggested to be unprecedented.
- The official Radio Times website with an increasing amount of Doctor Who content including archive material