Two young coders visit BBC TV Centre to find out how code is used in making Dr Who. We see archive Dr Who footage, and hear how the storyline used to be limited by the number of Dalek props that could be physically built. Science presenter Minna Kane tells us that modern episodes of Dr Who make extensive use of computer programming, particularly for controlling highly realistic visual effects (VFX) using computer-generated images (CGI). The children meet programmer Cat, who shows them how she can make an army of Daleks using 3D computer-generated images and a duplicate command in the code. She also demonstrates the RAND command, which can make the different members of her Dalek army move at random, making them seem more lifelike. They meet Rose, who films them acting in front of a green screen before using a chroma key effect to superimpose them into the Dalek Parliament. Teacher notes: Demonstrates how code commands can be used in creating visual effects. Could be used as an example of simulating physical systems as well as showing various forms of input and output.