Episode 24 of The Trigan Empire
The Thing from the Sea
aka The Menace from the Sea
Menace from the Deep (the Collection)
Dreiging uit het heelal (Dutch title)
Threat from Outer Space (Dutch title, translated)
The Thing from the Sea was originally published in 13 installments in issues 461 (14 November 1970) to 473 (6 February 1971) of Look and Learn magazine.
It is reprinted in the Don Lawrence Collection Volume 6, The Puppet Emperor.
 Disaster! The great liner Daveli, largest and newest passenger ship on the planet Elekton, is sinking on her maiden voyage!  Janno and his comrades dive to the bed of the Tibor Ocean to investigate the sunken wreck of the “Daveli”.  Following the mysterious sinking of the great liner “Daveli,” the mammoth Trigan Bay Bridge collapses!  Following the destruction of the Trigan Bay Bridge, it is feared that some malevolent intelligence is at work on the planet Elekton. While rescuing survivors from a wreck, Janno sees something strange . . .  A malevolent intelligence is at work on the planet Elekton. It takes the form of a monstrous mechanical device that inhabits the sea, and is able to destroy at will—slicing through metal like a giant sword.  A malevolent intelligence is at work on the planet Elekton. A mechanical monster from the sea is able to destroy anything, and a group of strangers has emerged unseen from the sea, among them Zatti, who has joined the Imperial Guards.  Janno and his comrades are investigating the mechanical devices which have been terrorising the planet Elekton. Janno is unaware, though, that one of the crew, Zatti, is an alien from the sea.  While investigating the menace of the monstrous mechanical devices from the sea, Janno and his comrades are captured and taken into a vast shape on the ocean bed.  Janno and his comrades are prisoners in the monstrous submerged conquest-machine, with which Ura Zircon and his people from the alien planet Thalla plan to take over Elekton.  Janno and his comrades are trying to seize the controls of the submerged conquest machine, with which the people of the alien planet Thalla plan to take over Elekton.  Janno and his comrades are on the point of destroying the conquest machine from the planet Thalla. Janno has only a few seconds to complete the operation before he is overpowered.  Having destroyed the conquest machine from the planet Thalla, Janno and his companions return to Trigan City with a prisoner. But the Emperor Trigo is in great danger from his Thallan secretary.  . . .
Plot summary (Dutch)