"City Under the Sea" is the fourth episode of the second season of Thunderbirds Are Go!, and was broadcast on the 5th November, 2016.
The Sullivan family, on holiday in the flooded Bay City, veer off the tour area and come face to face with the Mechanic, who is tearing apart the Creighton-Ward building. The Mechanic is looking for the plans to Parkmoor Scrubs prison, so he can free the Hood, but the debris from the crumbling building hits the Sullivan's sub and they crash to the bottom of the sea. Gordon and Kayo are sent out to rescue them, but things get tense when the Mechanic attempts to destroy another one of their machines. Lady Penelope, with Parker as chauffeur, uses her knowledge of the building to locate the plans first, but can they escape before the Mechanic notices?
- Virgil Tracy, Gordon Tracy: David Menkin
- Kayo: Angel Coulby
- John Tracy: Thomas Brodie-Sangster
- Lady Penelope: Rosamund Pike
- Parker: David Graham
- The Mechanic: Chris Jarman
- Ethan Sullivan: Sanjeev Bhaskar
- Emily Sullivan: Emma Tate
- Ava Sullivan & Lettie Sullivan: Tia-Lana Chinapyel
International Rescue Equipment Used
Non International Rescue Equipment Used
Gordon: "Go ahead, just say it."
Kayo: "Gordon Tracy, you are a secret slob."
Gordon: "Oh, the wrappers? What can I say? Scott's always at me about my room. This is my own personal domain."
Gordon: "But you don't even know how to operate Thunderbird 4... don't scratch it! And be careful! Ah, Brains is gonna kill me. Whoa, I don't even know it could do that!"
Parker: "Where to now?"
Lady Penelope: "Just follow my lead, Parker. I spent many a summer playing hide and seek in these walls."
Parker: "Bit different with a giant crab ship nipping at your heels."
Lady Penelope: "Don't worry, Parker. I always won."
- The TX-204 safe is a reference to the TX-204 target carrying aircraft from Trapped in the Sky.
- Unlike Chain Of Command, when Kayo boards Thunderbird 2 with Gordon, she has more animation than Alan and Scott when they entered the craft.
- Even though this is Kayo's first time in Thunderbird 4, she is able to control the craft. She is seen earlier in the episode to be watching what Gordon is doing while he is in the pilot's seat, making the assumption that she can learn skills on the fly.
- This is the first time fish can be seen swimming in the water of the ocean.
- Magna Prime can be seen attached to the Crab Ship.
- In the TX-204 safe, the files there shown are referenced XZ-157, same as the in-flight checklist in Deep Search and the ID number of the stealth mine in Space Race.
- This is the first episode where FAB1 uses all three modes (land, air, water).
- Chris Jarman is credited with voicing the Mechanic in this episode, but he is silent through the entire episode.
- Gordon mentions having broken a previous ship, which can be inferred as Thunderbird 4. However, TB4 would not be significantly damaged until Up from the Depths Part 1, two episodes later.