This is the first time Gallifrey is described in any detail. Susan talk about it to one of the Sensorites. It also shown that Time Lords have limited telepathic abilities. Susan is able to communicate with the Sensorites through telepathy.
The Tenth Doctor later confirms that the Ood are rather similar to the Sensorites as he mentions having once been to the Sense-Sphere which happens to be in the same system as the Ood-Sphere. (Basically, the Doctor Who series is the best at fan service.)
The TARDIS travelers land on a moving spaceship and find the crew apparently dead. However, one of the crew members, Captain Maitland, regains consciousness and Ian Chesterton fully revives him and another woman, Carol Richmond. These two tell the travelers that they are on an exploration mission from Earth and are orbiting Sense-Sphere. However, its inhabitants, the Sensorites, refuse to let them leave the orbit.
The Doctor works out that the Sensorites attacked the human craft because John, a mineralogist, had discovered a vast supply of molybdenum on Sense-Sphere. Susan (who is able to interact telepathically with the Sensorites) reports that the Sensorites want to make contact with travelers, asking the crew to go aboard Sense-sphere and reveal that a previous Earth expedition caused them great misery.
Susan, Ian, Carol and John join him while Barbara and Maitland stay behind. John is promised that his condition will be reversed. On their journey to Sense-Sphere, the party learn that the previous visitors from Earth exploited Sense-Sphere for its wealth, then argued. Half of them stole the spacecraft, which exploded on take-off.
Now the Sensorites are suffering from a disease that is killing off their kind. After Ian falls prey to this same disease, the Doctor discovers that the aqueduct has been poisoned by deadly nightshade. The Doctor is then able to help create a cure that saves Ian.
Meanwhile, investigating the aqueduct, the Doctor meets an unseen monster. Susan and Ian find him unconscious with a ripped coat, but otherwise unharmed. On being recovered, he tells of his suspicion that some Sensorites are plotting to kill them. The plotting Sensorites kill the Second Elder and one of them replaces him in his position.
Later, the Doctor and Ian go down to the aqueduct to find the poisoners. Their weapons and map were tampered with and are useless. Barbara and the others learn about the tampered tools and go into the aqueduct to rescue the Doctor and Ian.
Ian and the Doctor discover that the monsters were actually the survivors of the previous Earth mission, and they had been poisoning the Sensorites. it is decided that they will be sent back with Maitland, John and Carol to receiver treatment for madness. (Modified from Wikipeida)
Not an overly exciting few episodes. It was nice to see Susan actually be useful. I feel like Susan is just there half of the time with the Doctor, Ian, and Barbara carrying most of the show. I really wish I could like Susan, but I just can’t. She just tends to get on my nerves. She was alright in this one though.
There is a scene that I liked between Susan and the Doctor in which the Doctor berates her for not listening to him. Susan seems to be getting a little fed up with constantly having to do everything the Doctor says. She’s entering into her rebellious stage. While I do believe that the Doctor can be a little harsh, it does seem that most of the times that Susan decides to do something without consulting the others, it turns out badly. Of course she has good intentions, but I can see why the Doctor would just want her to fall in line. She’s almost gotten them killed a couple times.
I wouldn’t consider this one overly important. It had some good moments in it, but for the most part, I found it a little “eh”.