The Fifth Doctor and Amy land on a spaceship, amidst two species at war, on opposite ends of the timeline. They fight their war, and then, the next day, they fight their war all over again.
The Doctor and Amy may be trapped for eternity, and the appearance of an old friend may make the Doctor question his mission till the end.
- Bizarre Alien Sexes: Not that bizarre, the Teuthoidians are asexual technically, like Earth's slugs.
- Blob Monster: The Teuthoidian are gigantic bi-gendered slug beings that can devour humanoids whole.
- Brought Down to Normal: Amy and Zara are slowly turning human.
- Butterfly of Doom: The concept is discussed between the Doctor and the White Guardian.
- Call-Back:
- The Doctor implores President Astra to remember burly chap with the long scarf in the blue box.
- Romana mentions leaving E-Space, which has taken its toll on her body.
- Zara mentions having been imprisoned on Erratoon.
- Call-Forward:
- Beating out the release of "The Poison Sky" by several months, the Doctor recognizes his companion has been replaced, but lets her think it's fooling him.
- For the Doctor's personal future, Astra's resurrection by a powerful source of time energy that renders her immortal is very similar to what will happen to Jack Harkness in "The Parting of the Ways".
- Came Back Wrong: When the Doctor used the Key to Time to restore Princess Astra, it made her immortal by turning her into a beacon for temporal energy, draining the life out of all her subjects and aging them to death while she continued living.
- Cerebus Retcon: Romana's change into a form similar to Princess Astra takes on a new meaning in this story.
- Evil Laugh: When Romana reveals who the last segment of time is, the Black Guardian lets one out.
- Fearful Symmetry: The Black and White Guardians play a game of Bridge when they think all of their plays have cancelled each other out. Still they can't beat the other without outside influence.
- "Freaky Friday" Flip: Zara swaps her mind with Amy's.
- Heroic Sacrifice: President Astra takes the sixth segment of the Key to Time back into herself and dies in the process, saving Romana's life.
- Kirk Summation: The Doctor gives one to Zara and the White Guardian.
- Light Is Not Good: The Doctor stops Zara from giving the White Guardian all the segments of the key of time.
- Mood Whiplash: The Doctor pulls off a Heroic Sacrifice by jumping into the Chaos Pool with the Key of Time, dramatic music flares up, Amy and Zara shout out for him, the lake vanishes and ... the Doctor's stuck on a ledge.
- Mythology Gag: When the Doctor is surprised to see Romana, she quips: "You were expecting someone else?"
- The Nth Doctor: Princess Astra is in this story, from in the form from before she met the Doctor.
- Older Than He Looks: Zara has been observed as to be older than 190 years old.
- Previously on…: Snippets from The Judgement of Isskar and The Destroyer of Delights are played before the opening theme.
- Pull the Thread: The Doctor manages to suss out who Zara is quite quickly, but lets her think she's got him fooled. His Tenth incarnation would do something similar later on.
- Rapid Aging: Astra's people are being affected by a supposed virus that ages them to death. Unbeknownst to anyone, Astra is the source of it.
- The Slow Path: At least 190 years have passed since the Doctor and Amy last saw Zara. Probably a bit more if you take the events of The Prisoner's Dilemma into account.
- Time Stands Still: The Black and White Guardians are really fond of this trick in this trilogy.
- What Measure Is a Non-Human?: Amy (actually Zara in disguise) wants the Doctor to leave a ship of slug-like beings to their doom, and the Doctor says they have the right to live just as much as they do.
- Who Wants to Live Forever?: Astra's botched resurrection courtesy of the Doctor and the Key to Time has left her immortal, and seeing everyone she has every loved age and die has made her cold and cynical over the 200 years she has lived.
