Eight slash Q Panenka, a craggy comet with a 13km circumference, has an elliptical orbit that takes it between Earth and Jupiter. Which, in the year 2329, makes it a cheap means of space freight – the second class postal service of the solar system.

But when the TARDIS lands on Panenka, the Doctor, Tegan, Turlough and Nyssa discover a community falling apart at the seams – plagued by thefts, and mysterious disappearances among the 'piggybackers' who eke out a desperate existence on the comet.

While Tegan and Nyssa suit up for a dangerous excursion into the comet's Unstable Zone, the Doctor and Turlough find themselves pawns in a game that could lead to tragedy for both Earth and Jupiter alike…