Susan with the Sensorites

The TARDIS arrives on board a spaceship in orbit around a planet. Its three occupants at first appear to be dead. However, one of the crew members, Captain Maitland, regains consciousness and Ian Chesterton fully revives him and another woman, Carol Richmond. The two of them then tell the travellers that they are on an exploration mission from Earth and the planet they are orbiting is called the Sense-Sphere.

However, its inhabitants, the Sensorites, are refusing to let them leave. The Doctor and his companions realise that the alien Sensorites were the cause of the spaceship’s crew being in a state of semi-permanent paralysis. As The Doctor starts to investigate the Sensorites arrive on the spaceship. Not only do they re-freeze Carol and Captain Maitland once more but they also steal the lock mechanism from the TARDIS, thus preventing their return to the TARDIS.

Meanwhile Susan's telepathic mind becomes flooded with the many voices of the Sensorites trying to communicate with her. However, through this telepathic communication with Susan, the Sensorites convey a request from their leader for a meeting with the travellers.

The Sensorites and Carol

On their journey to the Sense-Sphere, The Doctor learns that previous visitors from Earth exploited the Sense-Sphere for its wealth. They then argued resulting in half of the human visitors stealing a spacecraft, which then exploded on take-off.

During the meeting with the Sensorites, Ian starts to feel ill. He starts coughing violently and then collapses. The Sensorites recognise that he is suffering from the same mysterious disease that has blighted the Sensorites since the earlier expedition left – a disease that always ends in death.

Ian's First Glimpse of a Sensorite

The Doctor is allowed to try and discover the cause of the deaths but his investigations into the cause of the sickness are hampered by the subversive activities of the devious City Administrator. It seems that the Sensorite Council is divided over allowing another party of humans to be on the Sense-Sphere. Some of the councillors believe that the humans can help with the disease but others continue to plot in secret to discredit the humans.

Eventually, after carrying out extensive tests, The Doctor discovers that Ian was actually poisoned by drinking water from the general aqueduct that has been contaminated with deadly nightshade. The Doctor finds the problematic aqueduct and starts work with the Sensorite scientists to create an antidote. The plotting Sensorites though capture and then impersonate a Sensorite leader, the Second Elder and steal the antidote before it is given to Ian, but when this is realised a new one is made easily and Ian is cured.

The Doctor then tracks down the culprits, who have caused the contamination, to a cave system near the aqueduct and uncovers three human survivors left behind from the earlier expedition, who have become deranged after experimenting with telepathy.

Meeting the Sensorites

When the human survivors are tricked into coming to the surface they are arrested by the Sensorites. The Doctor and his party then return to the city. The Doctor though pleads clemency for the poisoners. The leader of the Sensorites agrees and allows them to be retuned to their spaceship so they can then be sent back to Earth to be treated.

At the same time the City Administrator is arrested and banished after his attempts to cause mistrust between the Sensorites and humans has now been brought to light.