The Doctor, Leela and K9 are visiting Mars, sailing down one of the canals in the Doctor's houseboat. Mars has been terraformed by the Ares Corporation, the president of which is the Last Martian, the sole survivor of Mars' oldest indigenous species. The Last Martian is a cephalopod who was found at a dig site having been frozen for a billion years. Leela sees that there are dead fish in the canal, and K9 determines that they died from red bloom. The Doctor decides to investigate.