The TARDIS is travelling through the Vortex when it picks up a distress signal which Turlough recognises and, with Kamelion spasming strangely, tries to conceal from the Doctor. But as the Doctor and Turlough argue, Kamelion sets course for the source of the alien signal: Earth! And so is set in motion the story that will see the Doctor travelling on without either Turlough and Kamelion...
The penultimate TV serial of the Fifth Doctor is novelised by its original scripter, Peter Grimwade. To set things in place for the new Doctor, he has the task of clearing out accumulations from the Fifth's period (that's Turlough and Kamelion, Tegan having departed in the unnovelised 'Resurrection of the Daleks'), tying up their respective stories, and introduce a new companion, Peri. And tell a story that stands on its own in the meantime.
The result is a little mixed, but still quite reasonable. Turlough is given a substantial role in this story, and it is the second TV serial in which Kamelion actually does anything. Resolving their back-stories lays much of the story's basics in place. And, while I won't tell you exactly who, one of the Doctor's old enemies has a substantial role to play as well.
This story is inextricably tied with the show's history, and hence may contain elements that could confuse someone with no real familiarity with the show.