|Main enemy:||The Karadax|
|Part of:||The Scientific Secrets of Doctor Who|
|The Scientific Secrets of Doctor Who|
|All the Empty Towers||Silver Mosquitoes|
The Doctor gives Martha a plastic box to keep safe. They are in the caves of Karadax, and there is a Karadax octopus droid nearby. The Doctor dashes off to find it, and Martha opens the box to find a sock. Suddenly, a portal opens in the wall and out steps an older Martha. She tells the younger Martha that she should stop travelling with the Doctor, or people she loves like Letitia will be hurt. Martha realises this person isn't her, because Tish never goes by "Letitia". The duplicate reveals that she is an android from the Martha exhibit in the Museum of the Doctor. She was stolen and reprogrammed.
A Karadax steps from the portal and tries to take Martha. He has calculated that if Martha is removed from history, the Doctor will be unable to stop the Karadax. The android sends a jolt of electricity through the shark-like alien, sending him back to his time period, but also fatally damaging herself.
The Doctor returns through a crack in the rock, and the drone tries to break through. The Doctor gets the sock from Martha and throws it onto the drone's sensor grid. It overloads and collapses. The Doctor decides he should check up on the Karadax every few hundred years.
- The Karadax resemble sharks. They are less feared than the Daleks, Cybermen, or Draconians.
- The Museum of the Doctor is on Reldeen in the 45th century. The Reldeenii built it after the Doctor saved them.
- This short story accompanied a chapter on the laws of time.
- Martha recalls seeing Judoon, Plasmavore, (TV: Smith and Jones) and Carrionites. (TV: The Shakespeare Code)
- The android warns Martha the people she loves could get hurt, like her mum, her dad, Leo, and Tish. (TV: The Sound of Drums/Last of the Time Lords)
- The museum exhibit has Martha wearing a lab coat, (TV: Smith and Jones) a black military outfit, (TV: The End of Time) and a maid uniform. (TV: Human Nature/The Family of Blood)