At a refinery plant on the moon of Delta Magna, Supervisor Thawn returns from the home planet and is greeted by his co-workers Dugeen, Fenner, and Harg. Dugeen detects another ship that followed Thawn’s ship in to avoid detection. Thawn notes there’s a rumor that the Sons of Earth are planning to arm the local Swampies. He warns that the people on Delta Magna are starting to rally to the cause of the locals. Fenner notes that Rohm-Dutt, a gunrunner, may be bring weapons to the Swampies on behalf of the Sons of Earth.
The TARDIS materializes in the swamp and the Doctor and Romana emerge. The ground is too marshy for K-9 to accompany them. The Doctor determines they’re on the third moon of Delta Magna. Romana uses the Tracer and determines the sources indicates a very large range of detection: apparently the Tracer is malfunctioning.
Thawn and Fenner go out in a hovercraft to find Rohm-Dutt. They don’t know his exact appearance but he’s reputed to wear eccentric clothing. Meanwhile, Romana goes to higher ground to get a clearer reading while the Doctor makes himself a reed flute. Rohm-Dutt is traveling through the marsh with the Swampies and they capture Romana. Fenner and Thawn spot the Doctor, mistake him for Rohm-Dutt, and shoot him. Fenner realizes that the Doctor isn’t Rohm-Dutt and the Doctor reveals that Fenner just shot his head. The Doctor is sure that Romana was abducted and Thawn insists that he come with them rather than search for her.
The Swampies tie Romana up and Rohm-Dutt interrogates her. She claims she doesn’t know anything about the refinery and Rohm-Dutt figures someone sent her to investigate. She says she came with the Doctor but refuses to tell him anything about the fifth segment, raising his suspicions.
At the refinery, the Doctor explains he was looking for a friend but Thawn notes he’s intruding near a classified project. The Doctor recognizes it as a catalytic conversion plant, similar to hundreds he’s seen elsewhere. Thawn is surprised, since it’s a prototype, and the Doctor admits he’s from outside the constellation and has his own transportation. The Doctor easily proves his knowledge of the plant and casually suggests a major improvement. The plant workers still are not fully convinced and warn that the Swampies will kill anyone. An orbit shot is announced and Thawn takes the Doctor to the control center to watch.
Rohm-Dutt turns over the gun and ammo to the Ranquin, leader of the Swampies. He makes sure to get their “signature” and Ranquin provides the sacred Mark of Kroll. Rohm-Dutt says that Romana has said nothing and another Swampie, Varlik, suggests they sacrifice Romana to Kroll. Ranquin agrees with him.
Thawn explains that they shoot protein shipments to Delta Magna on a twice-daily schedule. The Doctor notices one servant, Mensch, but Thawn barely notices the Swampie. Dugeen is sympathetic toward the Swampie cause, noting that when they put in ten more refineries, the Swampies will be driven out. The Doctor notes that the lake couldn’t support that much protein extraction but Fenner assures him the surveys have shown it has sufficient capability. Thawn has lost two men earlier and the Doctor confirms they were killed in while taking samples, and the bodies disappeared. Thawn believes the Swampies are responsible but the Doctor isn’t so sure. The crew prepares for the orbit shot.
That night, Rohm-Dutt watches as Romana is brought forward for the sacrifice to the Swampies’ god Kroll. The orbit shot rocket passes overhead. Ranquin has the pit opened and calls forth Kroll. At the refinery, the Doctor tries to sneak away. Thawn and Mensch enter the departure chamber. They hear the native chanting and Thawn explains that Kroll is a giant squid that they shipped from Delta Magna when they forcefully evicted the Swampies and placed them on the moon. He doesn’t advise the Doctor to go out, as the Swampies are preparing for war. The refinery workers will use gas-powered mortars to dispose of them in the morning. Thawn leads the Doctor back to the control center but orders Mensch to stay behind. The Doctor slips back and sees Mensch send a signal out into the swamp. He slips out, steals a canoe, and paddles to shore.
On shore, a Swampie sees the signal and goes to the temple to report. The Doctor secretly follows him and watches as Romana is sealed within the temple. Tentacles reach for her as the chanting reaches its peak…
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