Dejected and physically tired following the departure of his granddaughter Susan,the TARDIS finds itself on the planet Dido. Upon exiting the TARDIS,Ian and Barbara find themselves atop a large cliff and find beneath them the hulk of a crashed space ship. Meanwhile inside the ship,a young lady named Vicki is attempting to send a transmission to a rescue ship to save her and fellow crew mate Bennett. An alien wearing a ritualistic mask than greets Barbara,than throws her off the cliff where she is found and rescued by Vicki. Meanwhile an avalanche traps the doctor and Ian inside the cave next to the cliff where they find an unknown force trying to keep them locked in. Barbara is awakened by Vicki,who introduces herself by explaining her ship crashed here and the rest of the crew,including her father were killed at a conference the Dido set up for welcome them.
Believing they were murdered,she has been tormented by Kolluquin,the alien who Barbara encountered whose determined to keep Vicki from leaving Dido,hence her secret transmission. The doctor and Ian soon escape,and encounter a crestfallen Vicki who just saw her "pet" Sandy unintentionally murdered by Barbara believing it to be a predator. The doctor later discovers a secret chamber where he encounters Bennett,who was masquerading as Kolluquin the entire time in order to escape rescue,as he was a prisoner and had killed his crew (save Vicki) and most of the Dido for the same purpose. After a struggle Bennett is killed while the doctor is sent back to the TARDIS by two surviving Dido,who had been a peaceful people. Realizing she had nowhere to do the doctor,Ian and Barbara decided to invite Vicki to join them and she formally accepts.
While an intriguing two part murder mystery that's very surprising up to the very end,this is also a very heartwarming character set up-with the majority of the story pairing up Ian and the doctor,and Barbara with Vicki. We see the doctor and Ian taking action to release themselves from the cave their trapped in. We also see Barbara trying to assist the confused and tormented Vicki,who lost her father than her only friend on Dido in her pet Sandy. Also this story shows us a very sensitive side of the first doctor. He shows calm tenderness and kindness to Vicki,and she is able to except Ian and Barbara-educationally inept by her 25'th century standards of course,largely through the influence of the time lord. An excellent introduction for an often unheralded companion.