A three-man rocket crew are nearly done with their mission to Titan Base until a course change puts the rocket in the path of a strange cloud in space. By the time they arrive, they have come under the control of a sentient virus which threatens the galaxy. When the TARDIS picks up an emergency message, it flies into the cloud, infecting The Doctor. To save himself and others, he must undertake a dangerous journey.
NOTES
This is not the only occasion where a Doctor shall encounter a clone of himself, in Project: Lazarus, the Seventh Doctor encounters an organisation called the Forge, who have created countless clones of the Sixth Doctor due to a cellular degeneration problem with their technology, transferring the memories of the previous clone to the next one so they believe themselves to be the same Doctor throughout.
This story also re-introduces the ‘old’ white console room (though slightly redesigned by Barry Newbery) rather than the ‘wooden’ secondary console room which debuted in The Masque of Mandragora.
Leela is left-handed, or at least writes with her left hand. Actress Louise Jameson is right-handed, but chose to make Leela a left-handed writer in order to increase her awkwardness at this task.
This is the only story in which the monitor on K9’s left side actually display
