The Ninth Doctor, Rose and Jack in modern-day Earth find a Neanderthal Man, 28,000 years after his race became extinct. The way the caveman time travelled means doing it again will kill him, so he has to stay in the present. While Jack teaches the caveman how to survive in the modern world, the Doctor and Rose travel back to find what caused the caveman to be transported.
- Affably Evil: The Hy-Bractors, who even ask if someone is a human before killing them.
- Arbitrary Weapon Range: Averted where a stray electromagnetic blast from an alien war in a distant solar system has hit a future Earth and long term disabled all electronics.
- Arranged Marriage: Rose has one arranged with Tillun.
- Call-Back: Das claims he went into a strange tree and was transported through time. In "An Unearthly Child" the TARDIS was described as a strange tree.
- Cannot Tell a Lie: The Neanderthals. It takes Jack some time to teach Das the concept of not being truthful. The Hy-Bractors also don't have the concept of lies. Oddly enough the first Hy-Bractor the Doctor meets knows humans tell lies but the Doctor is able to convince him he and Quilley aren't human.
- Comically Missing the Point: Rose says her grandmother married too quickly and her husband was carrying on with other women behind her back. Tillun tells her "When I carry on with other women, Rose, I promise I will do it right in front of you."
- Continuity Nod: The Doctor claims to have already met Neanderthals.
- The Doctor mentions the planet Celation, which sent one of the delegates in The Daleks Master Plan.
- Drugs Are Bad: The chemical combinations, which future people use to give them different emotions.
- Hoist By Her Own Petard: Chantal is defeated when the Doctor subjects her to one of her own drug combinations and she then tries to activate her own time machine because she's too mentally disorientated to recall why that won't work.
- It Only Works Once: Literally; the rip engine used to send the research team back through time causes such damage to the subject's genetics that any subsequent attempt to travel through time would kill them.
- Nobody Poops: Rose gets outed when pretending to be one of the future humans by saying she needs the toilet and being told that a tablet means we won't need to do this in the future.
- Underestimating Badassery: While Chantal manages to drug the Doctor to such an extent that she's able to perform a vivisection on him while interrogating him and then put him back together, the Doctor recovers from the drugs slightly faster than she was expecting, which allows the Doctor to take Chantal by surprise and escape.
- We Need a Distraction: The Doctor and Rose need a distraction to get through army security, so Jack volunteers "one of the biggest distractions you'll ever see," which turns out to be him running through the crowd naked. The Doctor laughs and says he's seen bigger.
