The Doctor's investigation into the nature and origin of the mysterious illness threatening the Moonbase.

The crisis escalates violently in the Moonbase control room when a Gravitron technician collapses with ominous black facial markings, signaling the virus's rapid progression. Hobson, already aware of Doctor Evans' …

Polly regains consciousness in Sickbay to find the Doctor examining a piece of silver paper, which he links to the Moonbase’s artificial lighting cycle—a fortnight of artificial day and night. …