Thawn, Fenner, Harg, Dugeen, and a green skinned native of Delta Magna are living on and working in a methane collecting--protein collection and manufacturing refinery. The green skinned man is called Mensch and he is one of the green natives of Delta Magna, kicked off their planet centuries ago by Earth colonists and given the third moon because it was thought unvaluable. Now, it seems the first of ten of these methane collectors will be built, driving the Swampies -- the nickname the colonists have for the green natives---out yet again. Thawn has returned from Delta Magna and gives Harg 500 microns, all they had in the library and tells Harg he doesn't owe him anything. Dugeen picks up something that followed Thawn's tracker ship down, having used Thawn's ship as a cover. The men discuss that the Sons of Earth may have given weapons to the Swampies here to use against the refinery. The Sons of Earth favour the rights of the green men even though many on Delta Magna have not seen a Swampie -- it was so long ago they were kicked off their home planet. Besides, Dugeen states, the Sons of Earth are known for their anti violence. Rohm-Dutt, a bounty hunter and known gun runner, has left Delta Magna, Fenner tells them. He read it on a news blurb. Thawn thinks it is he giving guns to the Swampies.

The TARDIS appears in long swamp reeds in the middle of a quagmire, leaving K9 marooned. The Doctor and Romana drudge through the swamp. The Doctor stops, sits, stands, and drops his hat and leg to do a gravity check. The pair deduce the gravity is low for a planet -- 1.5 miles per second needed to breakaway. The Doctor puts his finger in his mouth and takes it out, guessing this is one of the moons of Delta Magna and correctly stating it is probably the third moon. Romana stops as the Doctor holds his finger up, "Doctor, sometimes I don't think you're quite right in the head." The Doctor stands, "Never mind about that..." The tracer doesn't get a clear reading -- 42 and a half degrees, either it has a fault or the atmosphere has affected it. Romana tries higher ground.

Rohm-Dutt has a wide brimmed hat and looks like a nut case, dressed the part of one. Not unlike the Doctor. Thawn and Fenner go out on a hover glide car with Mensch to find the gun runner.

Rohm-Dutt is with the Swampies in canoes.

The Doctor uses a reed to play the flute music he loves so much. Two Swampies grab Romana from behind and she drops the tracer. The Doctor starts calling for Romana but Fenner shoots him from afar and he falls. Rohm-Dutt hears this and he and the Swampies, with Romana, leave in the canoes.

Fenner and Thawn see the Doctor on his face; Thawn knows it is not Rohm-Dutt. The Doctor gets up, Fenner just shot his hat. The Swampies, Thawn and Fenner admit, are getting bolder. Even though the Doctor insists he was part of a survey team, Thawn insists he come to the refinery with he and Fenner. The Doctor asks if there will be strawberry jam for tea.

Romana is tied by Swampies to a rock. Rohm-Dutt sits near her and questions her. She tells him that his emotional insulation is due to and indicative of psycho futile trama. Thunder starts and a storm rises. Rohm-Dutt knows they don't employ women at the refinery. Romana tells the truth about her partner -- the Doctor but explains she and the Doctor were here to catch butterflies. Rohm-Dutt threatens to choke her if she doesn't answer truthfully. The rising storm makes him stop.

Thawn pushes the Doctor into the refinery control room. The Doctor complains that they should put up no trespassing signs or prohibited area signs. This project is a secret. The Doctor remarks that the Earth colonies are so insular -- these refineries are in other places. He knows it is a methane catalizing refinery; he's seen 100s even though the men here insist it is the first ever built. The Doctor tells them that on the planet Benaca Benarda there is a refinery like this in every town. He answers questions regarding the vernicular gas separator and the primitive enzyme cycler. The swamps on the planet are bottomless and the men in the refinery think the Swampies killed two of their men. The Doctor wonders about that.

Rohm-Dutt gives guns to the Swampies but needs something on his papers. Ranquin, leader, orders the mark of Kroll, their "god" to be put on the paper. Skart suggests Romana be the sacrifice to Kroll. Ranquin agrees.

100 tons of compressed protein is shot into orbit. When the Doctor counts how many of them there are, the men do not count Mensch -- he is a servant, not a person. Since the bodies of the missing men are gone and were never found, the Doctor suggests it was not the Swampies who killed them. The Doctor subtley argues that the Swampies have a right to the land -- wondering if teaching them to be servants and carrying trays is something the Swampies should settle for. Fenner explains about progress. The Doctor smiles, "Progress is a very flexible word -- it can mean just about anything you want it to mean." As the men are busy with the firing of the protein for Delta Magna, the Doctor sneaks out of the room.

The natives chant Kroll over and over as drums play and the rocket shoots in the distance. The pit is opened. Ranquin calls Kroll from the bottomless deep. The ritual goes on and on.

In the refinery, the Doctor finds a flashlight on a ledge. Thawn and Mensch find him. They explain that the Swampies are having a blood sacrifice to Kroll, something Thawn believes the Swampies call a giant squid. To keep the natives happy, when the colonists brought them over from Delta Magna, they also imported some squid specimins. Thawn won't let the Doctor go.

The Doctor says, "Look, I have a shrewd idea who it is that might be being sacrificed -- I mean Romana's not the easiest of guests."

In one hour it will be light. Thawn will use this as an excuse to use the gas mortars on the settlement and wipe it out -- all in self defense of course. They go but the Doctor follows Mensch back, seeing him send a message via the flashlight to a Swampie out in the swamps. The Doctor sneaks out and this time, gets away using a canoe.

The Doctor smashes through the bushy fence, grabs the head off the "monster" which is revealed to be a Swampie, and punches the Swampie down. He then unties Romana, who feels she can see the Doctor's smug expression from behind. She yells at him to lookout as the Swampie recovers and flings a knife at them.

Dugeen find the shape on the bottom of the swamps stretches across the bay over two square miles and it has changed shape.

Ranquin tells Rohm-Dutt they are not going to hide from the dry foots' attacks this time. The dry foots are the whites, the men from the refinery. Rohm disagrees.

In the swamp, Romana tells the Doctor she dropped the tracer but he shows it to her, he picked it up. They wonder where all the protein the refinery is finding is coming from. Romana just wants to leave: it will be dawn soon. The Doctor finds a mine like shaft that goes to an underground passage. Romana hates underground passages and the Doctor admits he hates them also. Yet he goes down this one.

Thawn suggests a probe be sent into the bay bed when it seems something lifted off the bed and then re-settled. They realize the Doctor is gone and he took a boat. This suggests to Thawn that the Doctor doesn't know how to use a glider hovercraft or rejects technology which also suggests he is a Swampie lover -- he was too glib to be a Swampie hater. Thawn will go out and make sure the Doctor drowns.

The Doctor comes out of the shaft with a book bound in leather! It is an illustrated history of the natives--atrociously written but with pictures of a giant squid like thing with suckers--like marks the Doctor found in the ground of the swamp. The pair also figure this moon is like a reservation where the Swampies have been sent. The history or legend tells that Kroll awakened and killed everyone in the temple, finding the people indolent and fat, swallowed the symbol of its power, and even ate the high priest. Then it returned to sleep. Romana says, "Oh, I like a book with a happy ending." That was Kroll's third appearance and he is due for a fourth. The Doctor and Romana joke that Kroll is not one of those monsters that is always about the place, just pops up every couple of centuries. The Doctor drops the book back down the shaft and they go.

Ranquin tells another Swampie, Skart, to plant blood on the stone to make it look as if Romana was killed. Thawn, Mensch and another man arrive in the Swamp at first light; Mensch gets the Doctor's stolen canoe. The Swampies open fire on Thawn but one of the Swampie men gets the gun blown up in his face -- the bulky rifles are faulty. Just then, a huge tentacle rises up and wraps around Mensch, taking him toward a giant monster, no fake this time. Rohm-Dutt calls to Thawn as if he knows him but Thawn leaves without acknowledging him. Rohm-Dutt runs off.

Back at the refinery, Thawn thinks the Doctor warned the Swampies of their coming. Thawn frowns on the police or authorities coming -- once here, they will never get rid of the police. He wants to handle it himself. Fenner will not really want to kill all the Swampies once and for all but doesn't really speak up against Thawn either. Harg calls Dugeen up to the control room since the scanners seem to be malfunctioning. 12 and 14 are covered by some strange signals. Or the same signal. Dugeen comes up and finds that something is blocking them: Kroll. Thawn tells them the shape they now see is the monster he saw. Even though the scanners are some 400 yards apart, this thing is covering them.

Romana and the Doctor are near the huts of the Swampies and discussing Kroll. Kroll is the source of methane and protein. While it was napping, the two men who vanished from the refinery were taking samples -- Romana remarks it was like prodding a sleeping tiger. The feeding processes of Kroll must continue independently while it sleeps--probably through its tentacles. The heat exchanges account for the massive amounts of methane and protein. The Swampies arrive on the scene with Rohm-Dutt prisoner. The Doctor suggest to Romana that she introduce him as a wise and wonderful person who wants to help and adds, "Don't exaggerate." So, Romana says, "This is, uhmm..." But Ranquin order to seize them! The Doctor tells her, "I told you not to exaggerate." Ranquin tells Romana she will be sorry she didn't die on the Stone of Blood. He has to choose which of the seven holy rituals by which they will meet death by.

The refinery men talk about the monster. Fenner thinks it might not be hostile--it hasn't attacked them or been seen or shown itself for a long time that they have been here. Thawn thinks it is hostile and wants to try depth charges against it. That might just provoke it. Fenner wonders if they can poison it with cobalt radiation.

The Doctor runs down a list of tortures and deaths for Romana: fire, water, hanging upsidedown over a pit of vipers...she remarks that is only three. They will face one out of seven. Rohm-Dutt asks Varlik, one of the chiefs of the Swampies, for help. Varlik tells him that he gave them faulty guns... the barrels were corroded and the metals bent. The Doctor tells Rohm-Dutt he has the weight of history against him, calling Dutt a rogue and that he doesn't wish to die with a rogue. Romana doesn't wish to die at all.

Harg, at the refinery, listens to a strange sound. A tentacle off Kroll, breaks into the piping and starts to attack Harg.

Ranquin has found which of the rituals they will die by: the 7th. The Doctor smiles, "Seven, that's my lucky number!" When Ranquin goes on about Kroll, the Doctor calls out, "He's not the great one, he's the insignificant one." He goes on to tell them that if they are going to have someone impersonate Kroll, it might as well be convincing. Ranquin tells the Doctor that when Kroll's servants imitate him, they are doing his bidding and are as he. A part of Kroll. Varlik tells them that they saw Kroll rise at dawn before them. The Doctor tells Romana, "There's less time than I thought."

Harg is pulled out of the main pipeline where Kroll has put one of his tentacles. Thawn will not abandon the refinery even though Dugeen suggests one of Kroll's tentacles is comparable to one of the human's fingers.

The Doctor keeps admiring the architecture of the small tower he, Rohm-Dutt, and Romana are brought to for their death in the 7th holy ritual of Kroll. Early Samoan, he believes. He goes on about how old customs seem to survive and that he prefers gothic perpendicular as he, Romana, and Rohm-Dutt are tied to two wooden planks, one above their heads where their wrists are placed and one below, where their ankles are put. Varlik tells them he tried to get Ranquin to allow the Doctor and Romana to die by being thrown into the pit and covered with rocks thrown down by the Swampies but failed. Romana says, innocently, "Oh, thank you." Skart explains that the window above them is part of it -- as the vines and creepers that hold the wooden planks dry up, they will stretch the wooden pieces, pulling the planks, and snapping the spines of three victims. While this is told to them, the Doctor goes on about how he would have sacked the architect to which Romana remarks that if the Doctor is trying to take her mind off something, he is almost succeeding. He talks about the window to which Romana feels now she can die happy. The Doctor starts to tell her about some other adventure of his but she rants, twice saying, "I don't care!" This stops him.

Ranquin comes in and goes on and on about appeasing Kroll. The Doctor asks the secret of Kroll's power -- Ranquin gets close to the Doctor and tells him it was a holy relic brought by his people when they came here from Delta Magna many years ago. It allowed them to see the future. The Doctor says, "Good, I like to get things straightened out." Romana says, "I wish you wouldn't use expressions like that." Ranquin tells the Doctor, "Your mind is bent." After he goes, the Doctor tells Romana that Ranquin has narrow little eyes and one can't hypnotize someone with narrow little eyes. Rohm-Dutt tells them that Thawn paid him to sell guns to the Swampies -- it would have given Thawn an excuse to kill of the Swampies any way he wanted and with faked documents forged by Thawn and Rohm-Dutt, given the blame to the Sons of Earth. This would have discredited the Sons of Earth. Rohm-Dutt yells at the Doctor, asking him why he has to ask so many questions at a time like this. Romana asks a question of her own: why they call themselves the Sons of Earth. The Doctor thinks it a good question even though she doesn't care about the answer: none of the Sons of Earth could have even seen the Earth, in fact no colonist on Delta Magna has seen Earth. The Sons of Earth want the colonists to return to Earth. Rohm-Dutt believes that if the colonists returned to Earth, they would all starve. He tells the Doctor his ankles are breaking but the Doctor tells him it is just his imagination.

Thawn and Dugeen figure the monster Kroll is about one mile across or more, 140 feet high, and has 30 tentacles on one side alone. Thawn figures 35 depth charges should kill it. Fenner tells him that a missile strike from Delta Magna would take 8 hours and that is too long to wait to kill it. The strike would have to be low intensity but this plan is ruled out as too dangerous. A violent storm hits and they have to put the conductors up.

In the temple tower, on the bottom, the Doctor asks Rohm-Dutt how he's doing. As the storm picks up, the Doctor tells Romana that an electric storm on these moons can be quite spectacular. The trio are being stretched now and it is no imagination.

As the rain starts to hit them, outside, Varlik tells Ranquin and Skart (another priest but lower ranking than Ranquin), that the Doctor and Romana are not from the refinery and should be spared. They need the support of anyone not in league with the refinery men. Ranquin feels they need no one now -- they do not need the Sons of the Earth either. They have Kroll. Varlik questions this -- why did Kroll kill Mensch, who was with them. As the rain picks up, Ranquin tells him not to question Kroll and that Kroll will punish those who displease his servants -- such as Varlik is doing now.

At the refinery, Thawn watches the heavy rains through a hexagonal window. Rains block the scanners. Dugeen mentions if the winds get inside through the main pipeline which Kroll broke through, the inside could be blown apart.

The Doctor hopes for hail to break the window above which would allow rain in to wet the vines. He sings a note. Romana looks at him, "It's not that bad yet." He sings a pitch higher and the glass on the window breaks. Opera singer Dame Nellie Melba (1861 to 1931) used to do this as a party piece but she used to only break wine glasses. The Doctor tells them, "Pull, Rohm-Dutt, pull, c'mon, you too, Romana!" As the get up, we see the Doctor's rubber swamp boots. Romana comments that her nose just stopped itching as she got free, an example of displacement anxiety. The whole time they were tied up, it was itching. The Doctor urges them out before the rain lets up and the Swampies come out from under their umbrellas.

Kroll moves through the swamp as fast as he moved through the water and at 6 knots.

Ranquin and the other Swampies find the three victims gone and give chase. It is sunny again and as the Doctor tests the footing of the swamp area they are on, he hands Romana his hat and scarf, then jumps across. The other two follow him.

Fenner figures the monster is heading for the settlement and even though he doesn't like the Swampies, he doesn't hate them either. Thawn laughs, he doesn't hate the Swampies either but wants to see them removed and he thinks that Kroll, the monster, killing off the settlement (two miles away), is the best news he has had.

Kroll goes back down, following the fleeing Swampies. At the refinery, Thawn plans an orbit shot in five hours, too early -- to kill Kroll and the Swampies. A tentacle goes after the Swampies that are running away and many tentacles go through a grass wall and the gates of the Swampies.

Thawn plans to drop 1000 tons of hydrogen peroxide on Kroll by using the orbit shot, telling Dugeen that this will kill two birds with one stone. Fenner thinks Thawn mad: the already thin atmosphere doesn't lend itself to a fireball which might asphyxiate all of them.

The Doctor plans to stop Thawn, overhearing his plan as he and Romana enter the refinery undetected. The pair see the hole in the main pipeline. Thawn wants to kill all Swampies, calling them savages and uncivilized. Dugeen tells Thawn that there is no difference between them and the Swampies. The Doctor shhs Romana and they leave the control room area. Thawn calls the Sons of Earth cranks but Dugeen tells him that all life is sacred. Thawn hits Dugeen down and pulls a gun on Fenner.

The Doctor stops Romana from following him into the rocket cylo where he wants to try to stop Thawn from killing the Swampies...and Kroll.

Skart, Ranquin, and Varlik talk about Kroll.

Thawn has an override on the firing mechanism and the Doctor begins to enter the most dangerous part of the rocket cylo -- where he will be incinerated if it is fired, Romana reminds him of this. The Doctor says, "We'd better say goodbye now. Goodbye." Romana follows him into the cylo anyway.

Thawn orders Fenner to fire the primaries. It will take 50 more seconds to launch.

The Doctor is on a ladder sabotaging the mechanism on the wall, telling Romana to get out as she tries to coax him down -- telling him it is too late.

Dugeen gets up and hits a red button on the console of the refinery firing board but it does nothing...except make Thawn shoot him in the back. Dugeen falls, dead. Fenner calls it cold blooded murder and he will report Thawn.

The Doctor takes out a hammer from his coat pocket, "When in doubt..." he hits everything on the small box and the door on it closes and he falls. Romana yells, "Doctor!"

The firing is stopped. A fault in the primary ignition panel as Fenner reports to Thawn. Kroll submerges into the mud again, out of reach. Thawn tries to call his murder of Dugeen a justifiable homicide, believing Dugeen was a plant from the Sons of Earth. Fenner tells him just because he suspected that of Dugeen, that was no reason to murder him.

Romana is with the passed out Doctor, who is on the floor. The Doctor sits up suddenly, declaring he was out for seconds but Romana tells him it was minutes. The Doctor needed the rest he tells her but when he hears it is minutes, he gets up and they go out of the cylo, the Doctor telling her they cannot be seen loitering here. If someone puts two and two together and sees them there...Romana calls his attention to the fact that Thawn is there and holding a gun on them. The Doctor turns, "You're putting two and two together. I can tell you're putting two and two together. This is Romana." Romana says hello. The Doctor tries to bluff that they saw the door open and he figures to Thawn that is could have been the cleaning lady, "I don't suppose you have a cleaning lady, do you?" Thawn tells them to put their hands up. The Doctor says, "Haven't you forgotten something? Shouldn't you say -- don't make any sudden moves?" Thawn says it.

Swampies, lead by Ranquin, Varlik, and Skart enter the refinery. Ranquin will ask Kroll to destroy it all. Varlik once more questions Ranquin and Kroll's roles and why Kroll destroyed the village. Ranquin tells him that it because the dry foots (Romana and the Doctor) were promised to him but escaped the Swampies.

When Thawn brings Romana and the Doctor to the control room, the Doctor sees Fenner moving Dugeen's body and the Doctor tells him that wasn't necessary. Fenner tells him it was Thawn, who also finds no reason why he shouldn't kill Romana and the Doctor right now. Fenner tells him they need all the help they can get. The Doctor looks at the controls, "Oh look," he says happily, "It's coming this way." He also jokes that perhaps it wants to shake hands with Thawn or tentacles. He also annoys Thawn some more by telling him perhaps it wants to have him as pudding. As Thawn is annoyed as far as he can go by the Doctor and seems ready to kill him, a spear hits Thawn in the stomach and he goes down, dead. The Swampies enter and the Doctor puts his hands up, directing Romana to do the same.

Kroll's massive body hits the refinery, which is in the water on metal girder stilts. The Doctor asks Ranquin if he will give them death by the 8th holy ritual this time. Ranquin blames the Doctor for all these deaths. The Doctor tells Ranquin that Kroll cannot tell the difference between him and Ranquin or dandelions and bird dung. The Doctor facetiously watches Ranquin pray on his knees. Romana goes near a window before the Doctor can warn her but even when he does, she insists she is in no danger, Kroll doesn't know they are here, she claims. It does and a tentacles smashes through the window at her and nearly wraps her. The Doctor runs in and grabs her out and orders the door to the hall shut. It is.

The Doctor instructs Fenner to use the machinery to make as much noise outside as it can. "Shh, shh," the Doctor tells everyone. Kroll leaves and Ranquin moves out of the room. Fenner has the compressors on and puts on alarms, too. The Doctor puts his scarf around Romana's neck and goes to test his theory, using the tracer. He goes outside and to the top of the refinery landing.

Ranquin make a racket near the open main pipeline, talking to Kroll, wanting him to crush all dryfoots. A tentacle comes through and grabs him up, wrapping him. As Varlik and Skart try to help him, it pulls him out and kills him, thank goodness.

Through different struts, the Doctor sees parts of Kroll. He is about to approach a part closer, "Well, I've had a happy life. 760 -- not a bad age." A tentacle hits him, knocking the tracer down while another tentacles sneaks up behind him on the outer wall of the refinery. Yet another grabs him and wraps him up, dropping him to the floor as all the others. The Doctor reaches for the fallen tracer, which could fall through the grid to the ocean below. He manages to get it and touch Kroll with it. Kroll lights up and seems to vanish. Kroll is the 5th segment which is now on the end of what the Swampies call a stick -- a very special stick to Romana--the tracer. Romana tells the Doctor, "Well done," and puts his scarf back on him.

Another problem -- the firing bay is blocked, Fenner reports. With it blocked even though there is no methane, no Kroll, on protein (since it all came from Kroll anyway), the computer, designed to run by itself, will cause the shot to go off -- inside the cylo in about 50 seconds. The Doctor has no time to reconnect it and if he doesn't stop it somehow, the refinery will blow up in 40 seconds now. Fenner tells them the abort and the over ride are not working.

The Doctor watches as the countdown goes to 001 but as he does, he also pulls two large silver wires out of the console and puts them together, causing a large spark, hoping the two are the correct two. Afterward, the Doctor's eyes widen and then he droops. Romana asks, "Doctor, are you all right? You could have been killed." The Doctor thinks he is all right. This is the end of the refinery. The Doctor tells Fenner that sooner or later, someone will come for him from Delta Magna. He should try the natural life, perhaps teach the Swampies how to do a replacement for Kroll.