Episode Five, originally transmitted on BBC1 on 12 December 1991

To evade Inga and the other 'archaeologists', Marcie and Thomas have run into the pit dug by Miss Pendragon and her cohorts. In this supposed Celtic burial chamber the kids find a light switch and a set of concrete steps. Using torchlight, they investigate further.

Not far behind are Pendragon and her followers. She has been here before, many years ago; on a noticeboard are old newspaper cuttings featuring photographs of a much younger Pendragon.

Marcie and Thomas discover an abandoned office designated to Pendragon, where they find old files marked 'Ministry of Defence'. But then they are caught by Pendragon's people. Marcie escapes by ferreting up a ventilation shaft while Thomas is captured. Pendragon sees Thomas's (dyed) blonde hair and mistakes him for Aryan stock - he could be the new Chosen One.

Marcie discovers the files relate to project BHM - Behemoth! Meanwhile, Pendragon locates the Behemoth's control panel and reawakens it. As windows shatter in the school directly above, Pendragon's followers enter a large control room and see Behemoth for the first time - a large, antiquated machine. Pendragon reveals Behemoth to be a great machine mind... and a war machine.

Marcie re-enters the school via the gym hall air vents and finds Reet and Miss Maitland studying the school plans - the Behemoth device is clearly directly under the school.

Pendragon, who developed Behemoth until her extremist politics meant the project was axed decades ago, explains that the computer needs a human mind to complete its program - the Chosen One! Pendragon insists Thomas will go in the pilot's chair but as she stands in front of it, Thomas pushes her into the seat and binding metal arms clasp around her. A large headset descends on Pendragon, who says all is now ready for the visionary who has planned all of this.

Inga declares that the Behemoth must rise - the pilot's chair begins to ascend to meet with the mysterious leader who waits above. In fact directly above, standing on the gym hall stage, is Marcie who has deduced the Behemoth's precise location.

A black-coated figure enters the gym hall at that moment - it is Marcie's old enemy Mr Eldritch. He has arrived just in time to witness the Behemoth, with Pendragon aboard, crash through the hall's stage amid dazzling lights.