The name on the side of the freighter at the beginning of the episode is Kingston.

A previously unseen alien ship (possibly remote controlled) is featured in this episode. It launches from a submerged UFO and becomes airborne to fire a laser weapon at sea ships, destroying them.

The rescue plane that rendezvous with Skydiver 1 is SHADAIR Albatross 15. It also appeared in "Sub-Smash".

In this episode, the aliens have somehow clandestinely constructed a dome under the Atlantic Ocean from which to conduct seek and destroy sorties against Earth shipping.

Like "The Cat With Ten Lives", this episode features snippets of events from later in the episode after the opening titles instead of launching straight into Act I. Both episodes were written and directed by David Tomblin.

This episode reveals that Earth has at least one (probably more) manned deep space probes to aid in early detection of approaching UFOs. Deep Space Probe 87 (DSP-87) appears here, reporting the approach of 25 UFOs, but is destroyed by one of them.

Notice at 7:20 on the DVD that Lt. Anderson is wearing a zipped up jacket over his SHADO tunic to hide it from the civilian employees of Harlington-Straker Studios as he meets with Straker on a soundstage.

This episode reveals there is a space observation platform called Watchdog used by SHADO. It is apparently mobile, as Straker orders it to be moved to zone NML-12 to watch for movements of the amassed UFOs detected there by Deep Space Probe 87 before its destruction.

The documentary crew's underwater photographer is pulled out of the sea dead, the air line to his scuba tank cut. But who cut it? He'd reported seeing a strange, silver flying fish (the alien laser probe) moving towards him just before communication was lost. The alien probe is capable blowing up a sea vessel with its laser, but not, seemingly, such a fine-tuned job as slicing a thin air hose! Was there an actual alien swimming in the water as well? If so, for what purpose?

All of the divers in this episode wear "bite-down" oxygen pieces for breathing, so they shouldn't be able to speak underwater as they do!

The director of the fishing documentary remarks that the unusual appearances of warm water fish are occurring off the coast Cornwall. Cornwall is a county of England, making up the southernmost peninsula of the country.

A previously unknown Skydiver captain appears in this episode for his only appearance.

This episode reveals that Skydiver 1 (and, presumably, all Skydivers) carries "sea sleighs", small one-man underwater scooters that can tow a person through the depths.

Foster accompanies Straker on the underwater trek to the alien dome. In "Ordeal", Straker remarked (within Foster's dream) that Foster is an expert skin diver.

The largest number of UFOs described (though not seen) in any story of the series is in this episode, at 50.

After entering the underwater dome, it sounds like Foster says, "It's been a UFO," and the camera pans over a circular image in the dome's floor. Possibly it is meant to imply that a UFO landed on the sea floor and expanded a dome-like structure around itself, forming the alien underwater base in which Foster and Straker now find themselves.

From the stripe patterns on the walls, it can be seen that the same walls of the interior of the alien dome are used repeatedly from shot-to-shot as Straker and Foster make their way through the corridors!

The walls also waver a bit as Straker and Foster walk or run past them, indicating they are simple frames surfaced with paper, plastic, or cloth sheeting.

Apparently Straker isn't very observant. He should have known that the "Foster" he bumps into in the dome after separating from him earlier was a duplicate because he is wearing normal clothing instead of the wetsuit they both arrived in!

At 35:14 on the DVD, we see that the clock installed on Skydiver is a Secticon. Another Secticon model clock is seen in both "Flight Path" and "Survival".

At 45:29 on the DVD, we see the Lunar Module on the launch pad at Moonbase. But in later shots at 46:28 and 47:28, the Lunar Module is missing from the launch pad!

The scene of the UFO getting shot down over Earth and spewing orange smoke is borrowed from "Identified". 

MEMORABLE DIALOG

add a drop of castor oil.wav

a cup of Skydiver coffee.wav

in unknown country.wav

ever had that feeling you've been here before?.wav