[2] As the Moon drifts through empty space, the off-duty personnel of Moonbase Alpha gather in the Recreation Section for an evening of music.
Linked to a transmitting apparatus by electrodes, the four botanists sit holding hands around a table, giving the proceedings the appearance of a scientific séance.
In the experiment, Mateo hopes to isolate these brainwave patterns in his own mind, amplify them using the combined mental powers of the group, transmit them at plants sitting on the table and measure the response.
Their dispute is interrupted by a report that all of Alpha's sections experienced a momentary drop in temperature emanating outward from Hydroponics.
When she relates the details of her bizarre encounter, Koenig is hard-pressed to accept the conclusion that this was some sort of psychic experience.
Making his way to Hydroponics, Mateo experiences a series of eerie occurrences; the sound of footsteps in empty corridors, doors opening before he uses his commlock, and a sourceless shadow following him in a travel tube.
While threatening Warren's life, Mateo is horrified to see that his hand transformed in to a scarred and withered claw.
Bergman reveals that during each incident, in addition to the drops in temperature, Alpha's instruments have detected Mateo's electrical-wave pattern.
Bergman and Helena propose that Mateo, while experimenting with the most primitive, least understood segment of the brain, has tapped a latent paranormal power.
Some unknown aspect of his work has stimulated this area to a never-before-seen level of activity, resulting in a destructive force capable of killing.
The doppelgänger blames Koenig and company for ending its life and says it is a force of vengeance, seeking out those who collaborated in its destruction.
As they grapple, the spirit gains the upper hand and sadistically holds Mateo's face in contact with the sizzling energy field.
When turned face-up, John and Helana are horrified to see the face and body burned in the precise manner as the spirit his death has exorcised.
Supplemented by the regular Barry Gray score (drawn primarily from "Another Time, Another Place"), English musician 'Big Jim' Sullivan would compose an eerie, atmospheric composition for the sitar to serve as the primary music track.
Sullivan, a prolific session guitarist of the time, also appeared on camera—dressed in an Alpha uniform, he was the performer seen playing the piece in the story's opening scenes.
Script editor Johnny Byrne's idea for "The Troubled Spirit" arose from his desire to write a science-fiction ghost story.
[3] Both Byrne and production designer Keith Wilson recall that producer Sylvia Anderson was greatly involved in the creative development of this episode.
Shortly before production began, the Italian actor resigned, unwilling to commit to the twelve-month stay in England the role would require.
[3] The script contained several differences from the finished episode: (1) The music programme was written as a string quintet, of which Victor Bergman was a member—this was suggested by the presence of a violin in his quarters in the earlier episode "Alpha Child"; (2) Characters were to have spoken Mateo's given name 'Dan' throughout the segment; in the televised version, everyone only used his surname—including his girlfriend - the cast in fact pronounced it as "Matteo" sounds in Italian, i.e. "Matthew"; (3) In a scene cut for time (but glimpsed in the 'This Episode' sequence), Bergman demonstrated his method to destroy the spirit: energy matching the spirit's wave pattern, but with reversed polarity.
[5] The animated energy shimmer seen when the spirit vanishes and is absorbed into Mateo's body was the only new visual effect seen in this instalment; the shots of Moonbase were library footage.