The Doctor emerges from the TARDIS into a darkened room. Using his torch he sees a pair of thick rubber boots, a toolbox and a young woman in a swimming costume. He asks her where she came from because she wasn’t there a moment earlier but she replies with the same question. He tells her he has just come from the planet Pootle, famed for its beaches (and three metre tall gun-toting sharks). She tells him he is in Earth, in the British Academy of Sporting Excellence (BASE). A young man enters and addresses the girl as Emma. His name is Sid and he mistakes the Doctor for an electrician. The lights come on in the room, but the Doctor explains he didn’t fix anything. From a conversation between Emma and Sid the Doctor learns that someone called Laura has had a mishap.
As they leave the room the Doctor finds out that BASE is training elite athletes for the Globe Games. Sid is a javelin thrower and Emma is a swimmer. In recent days three of the athletes have died: Laura, a swimmer, in the pool; Joe, a sprinter; Andy, a discus thrower. The other athletes have covered up these deaths so that an investigation does not wreck their training regime.
Sid and Emma take the Doctor to a makeshift morgue where the three bodies are being kept. He discovers that all three died by electrocution. Their conversation is interrupted by the sound of marching feet and, upon investigation; they see four Sontarans in the corridors. The Doctor explains that they are aliens and their only weakness is the probic vent at the back of their neck.
The Doctor wants to get all the athletes out of BASE to safety. He surmises that the Sontaran-Rutan war is again spilling in Earth’s direction. The athletes take the Doctor to the common room where three other athletes - Karl, Jess and Holly – are still up. Despite their initial disbelief, the athletes follow the Doctor up to the bedrooms to warn the other athletes. They are too late; the Sontarans are marching the others down the corridor at gunpoint. Karl tries to escape through the fire exit but it is locked. Even the sonic screwdriver won’t release it. A chair flung at a window has similarly negligible effect.
The Doctor takes the five athletes back to his TARDIS. On the way the power goes out again. When the Doctor again trips over a pair of rubber boots he realises that the TARDIS is nearby. When he puts on his torch this time it is to discover a Sontaran in the room with them. Shining his torch into the Sontaran’s eyes to dazzle it, the Doctor tosses the TARDIS key to Jess. As she puts the key in the lock there is a flash of pale green light and she drops dead, electrocuted.
The lights flicker back on. The Sontaran holds the Doctor at gunpoint but he keeps up enough banter to distract the guard long enough for Sid to throw a screwdriver from the electrician’s toolkit into his probic vent. The Doctor puts n the rubber boots and some insulated gloves from the toolbox and reaches for the TARDIS. A spark leaps at him, telling him that his ship is still electrified.
Following the sound of screams they track the other athletes and Sontarans to the gym hall. Major Stenx is questioning the athletes to find out who and what they are. He is intrigued by their athletic prowess and calls for one man to demonstrate his prowess hurdling over a beam. When the athlete completes the task, however, he lands on an explosive charge and is killed. Stenx is smugly pleased; pointing out that athleticism is worthless without mental alertness. The Doctor attempts to rush into the proceedings, but Emma holds him back.
The Doctor and his four surviving friends return to the pool. The Doctor is perplexed that it is so cold but Emma says that the heating went off when Laura was killed a week earlier. A bad smell leads them to a grating where they discover a dismembered human hand. Suddenly an alarm blares and a Sontaran voice warns its colleagues that there are still uncaught humans abroad. Running for safety, Holly slips on the tiles and cracks her head. Sid falls over her, hurting her ankle. Emma dives into the pool so that when Captain Skeed and another Sontaran enter to apprehend the others she goes unnoticed. As the Doctor and the three athletes are marched through the building it is obvious from the Sontaran’s conversation that they have come to BASE specifically to look for something.
The Sontarans take their prisoners to the gym. Stenx is examining pieces of sporting equipment. When the Sontarans seem likely to kill the injured Karl the Doctor intervenes. He accuses the Doctor of being the shape-shifter that they are looking for. The Doctor replies that he is a Time Lord. Stenx smiles; capturing a Time Lord is a bonus for him. They lock the Doctor in a store cupboard. There is a knock on the door after a few minutes. Emma has found him, but the door is deadlocked. She tells him that all of the athletes are in the arena outside. Emma says that she is cross with the Doctor for saying that he was a Time Lord, but he responds by saying it was necessary to distract them from killing Karl. She wonders whether it would have been better to let Karl die instead of losing the chance to help all of the others. The Doctor responds by asking what would happen if it was Karl who had to save the world later. Suddenly, two Sontarans enter the gym.
There is no noise from Emma, but neither do the Sontarans catch her, even though she must be visible to them. Instead, the door is opened and the Doctor is led out of the cupboard and out into the arena. The stadium is brightly lit and the athletes are penned into two rows of seats in the stand. A shimmering in the air above tells him that a force field is in place above the arena. Suspended below it is a spherical mass: all of the javelins, hammers and discuses that BASE owns being kept safely out of harm’s way by the Sontarans.
The Sontarans have arranged a series of track and field events. The first is a hundred metre sprint where the Doctor is lined up against five other runners. Despite their prowess the Doctor feels that his Time Lord physiology will be enough to keep him out of last place and thus avoid the death that Stenx has promised the loser. However, during the race the Doctor decides to sacrifice himself and slows down. Karl has the same idea and the Doctor reaches back and drags him over the line with him. Stenx declares that they both finished last and should die together but the Doctor argues that the rule was the last man, not the last two, and to change the ruling would be dishonourable. His ploy works and Stenx, his honour questioned, relents.
The next event pits the Doctor against five long-jumpers. The rule is that all must pass a six metre line. The first athlete succeeds but the second falls short and is eaten by Sontar Sand Shrews. The next competitor refuses to jump and is shot dead, so the Doctor has to give a pep talk to the others to get them to jump to safety. The Doctor falls marginally short in his attempt, but the Shrews are foiled as they attack his feet because he is still wearing the rubber boots.
Next up is a tug of war against a giant robot. As the robot drags them ever closer the first of the human team is drawn across the red line and disintegrates in a flash of energy. The Doctor exhorts his companions to greater efforts and when he sees Emma emerge from the background he sets up a chant of ‘Swing Low Sweet Chariot’ but with the words altered to tell her how to switch off the robot’s gravity field. Emma climbs up the robot to do so, causing it to fly through the air and explode when it hits the energy field. Afterwards there is no sign of Emma and everyone assumes she perished along with the robot.
A side-effect of this event is that the four Sontarans have also lost their gravity and are floating above the ground and seem unable to aim their guns. The Doctor improvises a plan where his six surviving tug of war colleagues stretch the rope across the arena so that a team of gymnasts can make a human pyramid on top of it. At the top of this pyramid is Holly. Her task is to plunge the sonic screwdriver through a weak spot in the force field holding all the athletic equipment above the field. The plan nearly succeeds until Holly, still dizzy from her fall earlier, drops the screwdriver. However it is retained by Karl who throws it with unerring accuracy into the target. The equipment is released and the athletes use the javelins, hammers and discuses to kill the Sontarans.
The force shield around BASE dissipates and most of the athletes take the opportunity to flee into the night. The Doctor makes his way back through the empty building and finds Emma sitting by the TARDIS. She says that everything is sorted but the Doctor disagrees. He says there is still the problem of the missing electrician and the death of a swimmer called Emma. He says he worked it out by various clues: the hand by the pool, the time Emma stayed under water when the Sontarans arrived at the pool side, his knowledge of the Rutans dislike of heat and the coldness of the pool, and Rutans’ ability to absorb electricity and use it to kill. He tells her that is how he came to conclude that she is a Rutan.
Emma explains that her plan was to disguise herself as a swimmer and qualify for the Globe Games where she could trigger a major diplomatic incident to start a war. The war would escalate and wipe out the population, allowing the Rutans to occupy Earth before the Sontarans could take it for themselves.
The Doctor finds the dead electrician behind the TARDIS. Emma asks why he didn’t say anything when he suspected what she really was. He says that her helpfulness and genuine concern for the safety of others made him wonder if she was changing her nature. He offers to take her with him in his TARDIS, out of the war. She changes back to her true Rutan form, a ball of green jelly, and seems to be deciding whether to take him up on this offer or to try to kill him and steal the TARDIS when Major Stenx staggers into the room. Stenx fires several shots at her as she sucks the electricity from the building and unleashes it at Stenx. When this exchange is over two adversaries are dead. The Doctor barely has time to gain entry to his TARDIS before the energy in the dead Rutan is unleashed in a massive explosion, destroying BASE.
As he leaves, the Doctor wonders what decision ‘Emma’ would have come to.