Writer: Don Freeman
Artist: Stephen Dowling and Dick Hailstone
Published: 16th August 1947 - 24th January 1948 (F195 - G21)
Number of Episodes: 137
Scientists worldwide are researching the G-ray, following Lumiere’s discovery that it can be maintained with glaucosite, a mineral found only on the ocean floor, its purity increasing at greater depths. Glaucosite has been discovered off Crater Island in the Indian Ocean. Spurned by Garth, Karen elects to join her own government’s mission there. Soon afterwards Garth and Lumiere are similarly engaged by the British government.
En route with Dawn, their plane is attacked by another flyer and shot down over the ocean. Garth catches a glimpse of one of his attackers—a white bearded man. They are rescued by a passing British government ship, the “Research.” Captain Steel informs them that he is bringing scientific equipment for Lumiere to Crater Island. On arrival they find many nations on the same mission, and in the international saloon bar they come across Karen, who faints at the sight of Garth.
At the quayside Garth sees the white-haired man disembarking from a private yacht. Steel identifies him as Baron Von Grimm, a “harmless” hermit author studying folklore. Before Garth can challenge him, they learn that some presumed monster in the deeps has been decapitating divers. On his first undersea exploration Garth catches a glimpse of figures lurking in a marine forest, but is then attacked by a giant swordfish.
Meanwhile, Von Grimm encounters Karen who challenges his earlier story to her about witnessing Garth’s plane crash through mechanical failure.
Garth vanquishes the fish and returns to the surface convinced that the divers were killed by a human agency. Whilst Lumiere is assembling an apparatus to adapt Garth’s body to withstand pressures at greater depths, Garth follows Karen as she visits Von Grimm’s private estate. He pretends to be a writer, descended from the famous German brothers. Karen discovers he is lying, but before she can warn Garth she is badly concussed. Garth rushes her to hospital, then rejoins Lumiere, who adapts his body to withstand pressures at greater depths than other divers can go.
Garth descends to the deeps, but is drawn by a powerful current into a submarine cave beneath a volcanic island known as Moaning Isle. Here he discovers the wreck of a ship, before returning to the surface. News of his discovery reaches Von Grimm, whose men have been killing divers before they can discover the cave entrance. The wreck has a cargo of gold bullion, which Von Grimm is planning to salvage.
After Garth makes his third descent, the now-recovered Karen volunteers to receive Lumiere’s de-pressurising treatment, so she can warn Garth. In the deeps she aids Garth in battling with Von Grimm’s men, shooting one, whilst Garth decapitates another killer with his own sword.
Pursued by the “Research” Von Grimm’s yacht enters a cleft too narrow for the larger vessel to follow, so Garth and Karen swim ashore to locate its hiding place. They are ambushed by Von Grimm and his band of Lascars. In the ensuing struggle, Garth hurls Von Grimm into the creek, to his presumed death. However he survives and is picked up by his yacht, which returns to Crater Island. Here he recruits some unscrupulous characters from the other expeditions to aid him in salvaging the gold. He is observed by Dawn, whom he kidnaps and takes back to his estate, instructing his men go ahead of him to Moaning Isle.
Returned to Crater Island, Garth sets off for Von Grimm’s estate in search of Dawn. As Garth fights his servant, Von Grimm’s bungalow is set on fire, and Von Grimm escapes, carrying Dawn over his shoulder. Pursued by Garth, he plunges over the rim of an extinct volcano. Garth believes he has committed suicide—taking Dawn with him!
Synopsis by Philip Harbottle