Doctor Who: The Newbies Guide
Genesis of the Daleks
This is the fourth story in Season 12.
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. In Death to the Daleks she encountered the Daleks.
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 Daleks are a race of evil mutants encased in powerful cyborg bodies whose shape is reminiscent of pepperpots. The Doctor has fought them many times, first encountering them on in a city on their home planet of Skaro in The Daleks. During that story their home planet was highly radioactive, and the Daleks shared their world with a blond haired, blue eyed pacifist race of humanoids called the Thals. The Doctor considers the Daleks his greatest enemy and they are a serious threat to anyone they encounter.
The Time Lords are the Doctor's people. In The War Games they exiled the Doctor to Earth, allowing him to leave a few times in order to perform missions for them. In The Three Doctors they lifted his exile. In Frontier in Space and Planet of the Daleks they were willing to pilot the Doctor's TARDIS remotely to help him thwart a Dalek plot.
in The Sontaran Experiment the Doctor, Sarah, and Harry were investigating Earth to make sure if it was suitable for re-colonisation for the human race. At the end of that story they used a teleportation technology called transmat to return to the humans' space station. We did not see them arrive.
You might also want to have watched at least one Dalek story in order to appreciate the threat they represent.