The Brigadier tries to seal the giant maggots in the mine, but they burrow up.
The maggot kills a Global Chemicals employee, who had come to destroy the egg, and escapes before any analysis can be made of it, leaving the Doctor and Jones to analyse the trail of green slime it left behind. The Brigadier receives orders to seal the mine with explosives and clear the area using UNIT troops. The Doctor realises that this will only make the situation worse, but his plea to Stevens to allow an investigation is ignored. As the Doctor expected, the maggots simply burrow through and are now a threat above ground. Not to be intimidated by Stevens, the Brigadier sends Captain Yates into Global Chemicals, undercover as a Ministry official. The Doctor dons some improbable disguises, including a milkman and a cleaning lady, to get through the gates and move freely. Meanwhile, Jo has alienated Cliff, with whom she is falling in love, by ruining one of his slides. Determined to make amends, she heads to the sealed mine in search of a maggot to run some tests on. Having liaised with Yates, the Doctor learns that Stevens takes his instructions from the top floor of the complex, and heads there to find out who is in charge. The BOSS, or Biomorphic Organisational Systems Supervisor, turns out to be a supercomputer with its own megalomaniacal personality.