The sudden appearance of a dangerous gravity bubble causes the TARDIS to dematerialize aboard a ship of the Earth Empire on an emergency mission to deliver vital serum to a plague-ridden world. Before the adventure is over, The Doctor and his Companions must contend not only with the death-dealing gravity bubble, but with the ship's paranoid computer, space pirates, and an attack by androids as well.

Not mentioned on the back cover blurb is that this adventure prominently features the Doctor's old foes, the Cybermen.