Up From the Depths - Part 1.Deep in the Mariana trench a survey team encounter the TV-21, a prototype Thunderbird vehicle designed and flown by Jeff Tracy. When a systems failure causes the survey sub to be stranded on the seabed, International Rescue arrives to rescue the crew, but the Mechanic suddenly takes control of the surveyor and uses it to crush Thunderbird 4. Gordon escapes just as his ship is destroyed and must find another way to rescue the crew. Meanwhile, Lady Penelope and Parker investigate into how the Hood is communicating with the Mechanic.