Breakdown.While everyone is organising a surprise birthday party for Virgil, John directs his brother in Thunderbird 2 to a glacial ice shelf to investigate a 'breakdown' emergency call. Virgil quickly discovers that the lone figure he finds on arrival is part of a larger expedition trapped in a cavern beneath the ice. The expedition's leader, a scientist seeking a rare bacterium he desperately needs to cure his daughter of a serious disease, refuses to leave as the glacier begins to crumble, leaving Virgil no choice but to assist before time runs out.