A huge burrowing machine tunnels out of control at ferocious speed, cutting clean through to the center of the earth, to the twilight world of pellucidar. Once there, Dr. Perry and David Innes are threatened by half human creatures, lizard-like birds, and man-eating plants.

Lowly Gillingham Football Club are drawn against Premiership giants Newcastle United in the quarter finals of the FA Cup. A small group of 'Gills' fans decide to make the most of their 'game of a lifetime' by turning the journey north into a weekend away. On the eve of departure one of the group receives a visit from Terry, a man with no legs in a wheelchair, claiming he can get them a free minibus and free diesel. In return, all they have to do is take him with them!

The TARDIS arrives on the planet Marinus on an island of glass surrounded by a sea of acid. The travellers are forced by the elderly Arbitan to retrieve four of the five operating keys to a machine called the Conscience of Marinus, of which he is the keeper. These have been hidden in different locations around the planet to prevent them falling into the hands of the evil Yartek and his Voord warriors, who plan to seize the machine and use its originally benevolent mind-influencing power for their own sinister purposes.