Tonight's aquatic adventure kicks off at a base deep beneath the sea, where scientists have completed drilling through the Earth's crust to find out what lies directly beneath. Boss Mr Prescott impatiently awaits the results: "Come on Andrews, don't keep me in suspense, what's beneath the Earth's crust?" "Seawater, sir. Plain, ordinary seawater." "We spent years drilling through silt and rock just to find more sea?!"
The clearly disappointed Mr Prescott is eventually convinced that it could be an exciting find after all: who knows what strange things might inhabit the Subterranean Sea?
At Marineville, Phones, Troy Tempest and Atlanta Shore are all dressed up and ready for a tropical holiday (Marina's only concession to holiday wear is a few pearls in her hair), but Commander Shore has some bad news: "I'm cancelling your furlough, Captain." He wants someone to go and check out the mysterious new sea, and it seems only Troy and Phones are up to the job.
So, changing back into their uniforms, they head on over to have a look (Marina leaves her pearls in). It turns out that soundwaves won't carry from the area beneath the earth's crust, so on its new voyage Stingray will be totally incommunicado.
Stingray heads down to the murky depths, where at first there's total darkness, then light so bright that Troy has to put his shades on (Phones obviously didn't think to bring any).
The rocks all glow with pretty colours - which would, of course, have been lost on British viewers having to make do with black and white. In fact, the conspicuously dull Subterranean Sea has little going for it except its proto-psychedelic palette, so for anyone starved of that it must have been quite a slog.
Stingray's propelled forward by an incredibly strong current. Eventually it crashes, rendering the crew unconscious.
When they come round they find the ship's barely scratched, but are stunned to discover that the water has disappeared and the area outside the ship is now a desert. They pop out to have a look.
A (real) tiny green lizard can briefly be spotted peeping out from the rocks.
Marina alerts Troy and Phones to a strange roaring noise - it's the water, coming back. Our intrepid trio leg it back to the ship.
Speaking of legs, the ones we see being splashed with water definitely don't belong to puppets.
The water returns so quickly our heroes have to swim the rest of the way to their craft, but they eventually make it back to safety, the only real casualty of these events being Marina's hairdo.
Deciding the explanation for the coming and going of the sea is just a particularly extreme tide, the crew of Stingray head back. But instead of coming to the surface where they start out, they emerge right by a paradisical island complete with guitars, cigars and an obligingly hula-ing native girl. Atlanta's consumed with jealousy on seeing the snaps. As is her wont.