Moon Graffiti
by Dave Stone

Seeking a pocket sub-dimension with which to recalibrate the
damaged TARDIS, the Doctor takes Peri to Earth in the far future after an invasion by Pararachnoids. The swarm has now departed Earth, leaving behind only the weak and crippled of its race -- although even they are still capable of tearing a human limb from limb. The Doctor and Peri encounter a young man in the ruins and are attacked by Pararachnoids, and the Doctor gives himself up to give Peri time to get to safety. She takes with her the glowing disc the young man was carrying, which turns out to be a tiny ship populated by a species of explorers called the Wibbly-Wee. Unfortunately, she is then captured by the monitor robots sent to return the young man to his place on the Line -- a conveyer belt of humans in suspended animation, who retreated underground to escape the Pararachnoid invasion. Due to a fault in the system the Monitors have concluded that the best way to protect their charges from the Pararachnoids is to keep them here, where they will never encounter a Pararachnoid. The Wibbly-Wee woke young Chimoani from his slumber hoping he could carry them to the materials they needed to repair their damaged ship. The Doctor and Chimoani manage to escape from the Pararachnoids, but they are followed back to the Line, which to the Pararachnoids seems like a giant buffet; and Peri has now been installed in Chimoani's place. The Doctor and Chimoani take the Wibbly-Wee ship to the control room, where the Line's organic operator died of a heart attack some time ago. Chimoani takes his place, and is able to reprogram the Monitors to drive off the attacking Pararachnoids. He then awakens the remnants of the human race. The Wibbly-Wee help the Doctor to repair the TARDIS before going, and he leaves the human race to begin rebuilding.

-Placement: Immediately after Vengeance on Varos, since the Doctor refers to the acquisition of zeiton-7 ore as only the first step in repairing the TARDIS.