A giant heavy metal extraction plant, moving along the Pacific seabed and collecting toxic waste, has a sudden compartment fire, leaving it out of control and heading for a undersea chasm. Thunderbird 2 is dispatched with Thunderbird 4 to effect the rescue of the only crewman, Ned Tedford once again, and make safe the toxic waste it is carrying. Lady Penelope, meanwhile, investigates the company that owns the extraction plant, when both John and Gordon suspect the owner was more concerned for the cargo, than the crew, and soon has trouble from the Hood.
