Doctor Who: The Newbies Guide
The Sontaran Experiment
This is the third story in Season 13.
The Fourth Doctor dresses in a rather bohemian outfit, notable for its long scarf. Unlike his previous incarnation, he is not willing to base himself on Earth with UNIT.
Sarah Jane Smith is a journalist who, in The Time Warrior, sneaked into UNIT in search of a story and ended up accompanying the third Doctor on a trip to the middle ages. Sarah has subsequently become the Doctor's travelling companion.
Harry Sullivan is a naval Doctor working for UNIT. In Robot he was responsible for examining the newly regenerated Fourth Doctor. At the end of that story the Doctor offered Harry a trip in the TARDIS. Harry accepted, not knowing that the TARDIS is actually capable of time and space travel. Harry is something of an upper class gentleman.
Recurring and Historical Characters
The Sontarans are a warrior race who are involved in a very long-lasting war with the Rutan. In The Time Warrior the third Doctor and Sarah defeated a lone Sontaran called Linx who was using crude time travel technology to bring human scientists from the 20th Century to repair his spaceship, which had crashed in medieval England.
In The Ark in Space the Doctor, Sarah, and Harry landed on a space station which contained a group of humans who had gone into hibernation to survive a global catastrophe. They had overslept, and during that story the Doctor woke them up. At the end of that story they agreed to transmat down to Earth's surface in order to check whether the planet is suitable for recolonisation. This story begins when they arrive there.