Tracking their latest clue to the location of a CVE, the Doctor, Romana, Adric and K9 arrange for a prisoner on an isolated planet to be released so that they can investigate his story that he witnessed a comet vanish when it passed a certain part of space. However, when investigating Regis's findings, the TARDIS crew discover evidence that some vast object is throwing gravity in the local solar system out of kilter, to the extent that it is disrupting the orbit of nearby planets. Taking Regis in the TARDIS to investigate, they discover the world-ship of the Engineer, a brilliant yet twisted scientific genius who seeks to travel into the universe of N-Space as part of his quest for new scientific understanding, but doesn't care what he does to other worlds in the process.
Trope:
- A.I. Is a Crapshoot: The Engineer relies on robotic enforcers to keep his world-ship in operation and dismisses K9 as just a 'thing', which allows the Doctor to reprogram some of his Enforcers to help him and his companions later.
- Alas, Poor Villain: The Doctor at least offered to save the Engineer after learning why he was on his quest, but accepts the Engineer's decision to stay with his wife's body as the ship explodes.
- Berserk Button: The Engineer flies into a rage when Romana and K9 just try to conduct a scan of the strange black cylinder he identifies as his 'treasure'.
- Better to Die than Be Killed: The Engineer's people apparently operated on this principle, with their forces often destroying themselves when they seemed about to lose a battle, and the Engineer's wife being killed when his planet's leaders destroyed her city when it was taken by their enemies.
- Body Horror: The Engineer used the body of Anla's son Jonas as part of his experiments to bring the dead back to life, restoring Jonas as a cyborg with an independently programmed brain.
- Cassandra Truth: Regis Tel is in prison for trying to warn his planet about the coming of the Engineer's world-ship.
- Everyone Calls Him "Barkeep": The Engineer insists on only being referred to by his self-proclaimed title, although the Doctor, Adric and Anla eventually deduce that his name is Baltazar Cayden of the Halacon Empire.
- Evil Cannot Comprehend Good: The Engineer assumes that Romana's advanced intellect means that she will be able to "understand" his experiments to reanimate Jonas's body as a cyborg, and assumes that residents of N-Space wouldn't care about any planets in E-Space, incapable of comprehending why people capable of understanding his scientific achievements would condemn them as horrible.
- Healing Factor: Adric uses his accelerated ability to heal to help in an escape from captivity.
- Hero of Another Story: Alna Jessik tried to arrange her own rebellion against the Engineer some time before the TARDIS came to the world-ship.
- It's All About Me: The Engineer continually acts as though everyone else in the universe is only fit to be raw fuel or slaves to help him sustain his ship, and anyone who disagrees with his goal or tells him that his dream is impossible 'must' be wrong just because they don't agree with him. At one point, rather than spend months searching an area of space for a CVE, the Engineer destroys a planet's moon with the intention of tracking the remains to find the CVE, even after he's spent thirty years searching and could surely wait a few months.
- MacGuffin: The Engineer's Treasure is a stasis chamber containing the body of his wife, who he believes he will be able to bring back to life.
- No Endor Holocaust: Basically defied;
- The Engineer destroys a moon to try and find the CVE by tracking the debris, and characters are panicked at the notion of the damage that destruction will cause, although it is suggested that most of the debris drifted away from the planet.
- Later the Engineer's world-ship is destroyed, but it may have been far enough away so that its destruction didn't endanger the other planets, or its gravity power source helped to contain the debris.
- Noodle Incident: Romana mentions that the TARDIS has spent months searching E-Space for another CVE.
- Normal Fish in a Tiny Pond: Inverted; the Engineer believes that he has basically exhausted anything the science of E-Space has to teach him and now seeks to travel to N-Space in the hope that he will be able to learn more to achieve his goal.
- Planet Spaceship: The Engineer proclaims that his world-ship would have been the first of many if his people hadn't lost the war just after it was deployed. The ship can reconfigure its outer appearance to appear a dead rock or a more lively planet, has engines at its core, and possesses powerful gravity weapons that are capable of destroying a moon and damaging larger planets.
- Saying Too Much: Adric nearly does this when he admits that he's from E-Space; the Engineer had been keeping him alive in the belief that he would be a source of information on N-Space and almost kills him before the Doctor sabotages the ship.
- Sole Survivor: Anla Jessik was a major military leader on her world, and is now apparently the last survivor of her civilisation because she was kept chained up as an example after a failed rebellion.
- Underestimating Badassery: The Engineer immediately assumes that the Doctor is dead after being caught in a rockfall, but Romana never has any doubt that the Doctor will survive and strike back. Even after learning that the Doctor is alive and active, the Engineer never considers the Doctor a serious threat
- Woobie, Destroyer of Worlds: The Engineer has destroyed countless worlds over the last thirty years, in the name of trying to find a way to bring his wife back from the dead.
