The TARDIS arrives on the unnamed planet of the Gonds, who are ruled and taught in a form of self-perpetuating slavery by the alien Krotons - crystalline beings whose ship, the Dynatrope, crash-landed there thousands of years ago after being damaged in a space battle. The Krotons are at present in suspended animation, in a crystalline slurry form, awaiting a time when they can be reconstituted by absorbtion of mental energy. Periodically, the two most brilliant Gond students are received into the Dynatrope, apparently to become ‘companions of the Krotons’ but in truth to have their mental energy drained, after which they are killed.
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Episode 1
The TARDIS lands on the planet of the Gonds, a primitive and enslaved
people. Every year, two of the finest Gond students are sent to be
'companions' of the mysterious and unseen Krotons. But the Doctor
reveals that only death awaits them.

Episode 2
Zoe and the Doctor submit themselves to the Krotons' IQ tests and are
selected to enter the Dynatrope, the Krotons' ship. There they watch
as the crystalline Krotons are brought back to life.

Episode 3
The Doctor and Zoe are pursued by a Kroton as they attempt to return
to the TARDIS. The Gonds prepare to destroy the Krotons' ship, but
Jamie is still held prisoner inside.

Episode 4
The Krotons are preparing to leave the planet and demand that the
Gonds hand over the Doctor and Zoe, so that their mental energy can
be drained.