Sarah Jane is woken by noises from her living room, and investigates to find that she's being burgled by a timid middle-aged woman dressed like a librarian. The woman demands all of the gold in the house, but when Luke enters the living room, also disturbed by the noise, the woman hesitates and flees, rasping "Our thoughts shine." The next day, Luke and Maria keep an ear out at school, and Maria overhears Julie Price confiding in a friend that she caught her mother Dana with gold jewellery that had been stolen from the girls' changing rooms. Sarah sees a news report about a 70-year-old woman, Hilda Sheen, who was arrested for stealing gold; when she investigates, she learns that Mrs Sheen tried to slip away from police custody with more stolen jewellery, and was picked up again near the Auriga Clinic in Hounslow. Dana Price has also been visiting a clinic, and since Maria strained her back playing netball, Sarah decides to take her to the Auriga Clinic, posing as her aunt.
The clinic is run by Dr Francis Augur, whom Sarah profiled for Metropolitan Magazine in 1987. She gets an interview by reminding him of their previous encounter, and as she and Maria enter the clinic, they see Dana Price walking out. Dana is the woman who tried to burgle Sarah's house last night, but she doesn't recognise Sarah at all. While Maria visits the clinicians, Sarah interviews Augur, who explains that he injects his patients with particles of gold and titanium and then runs a small electric charge through their bodies, eliminating stiffness and fatigue by generating warmth and restoring the balance of ions in their bloodstream. According to Augur, gold is non-toxic, does not react with other chemicals, and could even be used to eliminate pollution in the world's oceans if used properly. Sarah jokes that he wants to save the oceans in order to harvest their gold, but he claims that there isn't enough to make it economically feasible with current technology. He then hesitates for a moment, takes a small patch from a box on his desk and presses it to her neck, saying, "Our thoughts shine."
Sarah tries to question Augur about the patch and his clients' odd behaviour, but the irritated Augur calls an end to the interview, claiming that the patch is for an unrelated medical condition and dismissing Hilda Sheen and Dana Price's kleptomania as irrelevant. Sarah leaves his office, and is appalled to learn that Maria has been given an injection -- without charge, but without Sarah's consent. Maria claims to feel fine, but Sarah takes her straight home and has Mr Smith, her super-computer, scan Maria for signs of alien infection. He detects nothing odd about her, apart from the gold particles in her bloodstream, which will naturally pass out of her system within seven days as Augur had claimed. Maria tells Sarah that, while waiting for the clinicians to arrive, she found a workshop full of smelting equipment behind the treatment room -- and that the nurse who gave her the injection said "Our thoughts shine" as he did so.
Clyde decides to tell his mother that he's staying with a friend and stake out the Auriga Clinic overnight. Sarah makes Luke promise that he won't do the same, but her wording is imprecise and Luke keeps to the letter of his promise by showing up outside the clinic at the break of dawn. Clyde admits that he fell asleep at one point, but he did get several photographs of people walking in and out of the clinic all night, including Mrs Price. He also took a blurry photograph of something like a gleaming gold robot climbing down the clinic's outside wall. To his and Luke's surprise, Maria then walks out of the clinic, presumably having let herself in while Clyde was asleep -- but when they approach her, they see that she's entranced, and she walks away from them saying, "Our thoughts shine." Luke and Clyde report to Sarah, who is furious with herself for putting Maria in danger. When Maria arrives to go to school with Luke and Clyde, she has no memory of having left her home the night before, and Sarah warns Luke and Clyde not to tell her until they know for sure what's going on.
While the children are at school, Sarah instructs Mr Smith to scan Maria again tonight, give her a clean bill of health whatever he finds, and then report the truth to Sarah. Mr Smith also scans the skies above London, and finds a golden satellite in geostationary orbit, transmitting signals to the Auriga Clinic. Mrs Sheen has fallen into a coma and is in police custody, and Dana Price has disappeared, forcing the distraught Julie to stay with her aunt. Mr Smith scans Maria as requested, and informs Sarah that the gold particles in her system have received an alien transmission and shifted into her optic nerve. Theorising that the particles could transmit as well as receive, Sarah orders Mr Smith to keep monitoring Maria and to hack into any subsequent signals he detects. She feels guilty about putting Maria in danger, but knows that she has little choice if she's to find out what's happening.
Clyde goes home, exhausted by his overnight stakeout, while Luke and Sarah wait for something to happen. Just after 11:00 pm, Mr Smith picks up a signal, and hacks into it to reveal that the gold particles in Maria's body are transmitting visual data from her optic nerves. Watching through Maria's eyes, Sarah and Luke see their friend walk out of their home, get into a car driven by Dana Price, and travel to the Auriga Clinic. There, Maria joins several other people in the workshop and laboratory that she reported seeing earlier; it's difficult to make out what's happening, but they appear to be conducting experiments on ordinary water. Luke texts Clyde to let him know what's going on, and he and Sarah head for the Auriga Clinic to investigate in person.
Sarah uses her sonic lipstick to get past the security cameras, but while exploring the clinic, she and Luke are attacked by the golden robot that Clyde saw earlier. Augur arrives just in time to stop it, using a control disc pressed to his forehead, but he accuses them of endangering themselves by interfering in his experiment. He admits that he is working with an alien race, the Keratin, who use gold to conduct and amplify their psychic powers; however, he insists that he and the Keratin are equal partners. The Keratin need his help to conduct certain experiments in Earth's environment, and in exchange, they have provided him with the technology to create a remote-controlled golden exo-suit, which can be used to conduct complex and delicate operations in environments too dangerous for human beings. Augur insists that the Keratin are simply communicating their wishes via the gold particles in his patients' blood, and not controlling their minds; however, he admits that they can become "over-enthusiastic," which is why he's been taking lithium concentrate patches to block their influence. Sarah's only question is whether Augur is under the Keratin's control or simply fooling himself.
Augur takes Sarah and Luke to his office to meet the Keratin's representative, a slug-like alien wearing a crown of golden implants. But when Augur repeats that theirs is an equal partnership, the alien bursts out laughing and reveals that it's been capable of speaking English all along. A tendril lashes out of the Keratin crown and injects more gold particles directly into Augur's neck, overwhelming the chemicals from his patch. Two more tendrils lash out at Sarah Jane and Luke, and the Keratin marches them back to the laboratory to put them to work with its other human slaves. However, Clyde then bursts in and slaps Augur's patches onto Sarah's and Luke's necks, and they flee to the lobby with the enraged Keratin in pursuit.
Clyde explains that Luke phoned him as soon as Augur captured them, and that he was listening in on everything that Augur and the Keratin subsequently said. He made his way to the clinic and slipped in behind Mrs Sheen, and he stole the box of patches from Augur's pocket before setting off to rescue his friends. Sarah realises that the Keratin has stopped pretending to cooperate with Augur because its research is complete, but then the Keratin catches up to them and she and the boys are forced to split up. Sarah runs back to the laboratory to rescue Maria, but the Keratin pursues Luke and Clyde, and chases them into a dead end. Clyde reveals that he also stole the control for Augur's exo-suit, and Luke presses it to his forehead as the Keratin draws closer.
The Keratin's human slaves overpower Sarah when she reaches the laboratory, and Maria informs her helpless friend that the Keratin's slaves have developed a bacterial culture that will boil the Earth's oceans away within a day, killing the planet but exposing 20 million tonnes of gold with which the Keratin can build remote-controlled battle machines to continue their war of conquest across the stars. But before Maria can inject more gold into Sarah, the exo-suit arrives and smashes down the door, distracting the slaves long enough for Sarah to press patches to Maria's and Mrs Price's necks. Clyde and Luke are close behind the exo-suit, which Luke is controlling; he managed to fight off the Keratin, injuring it, but Sarah realises that it's heading for a communicator to transmit the formula for its bacterial culture to the rest of its kind. However, she also realises that it chased Clyde and Luke instead of her, retreated when they headed for the laboratory, and used human slaves to do its work for it -- because it's a slug, and the salty seawater it's been using for its experiments is lethal to it.
While Mrs Price looks after the other recovering human slaves, Sarah and her friends head back to Augur's office carrying jars of seawater. The Keratin has activated a communications unit hidden behind a false wall, but Sarah shorts it out with her sonic lipstick. The Keratin attacks, shrugging off the seawater that Sarah's friends throw over its body -- but before it can kill Sarah, Augur rises from the floor and jabs an injector full of seawater through the Keratin's hide, injecting the salty water directly into its bloodstream. The Keratin convulses and explodes into a cloud of foul-smelling smoke as the shaken Augur watches, finally accepting that he's been used. Sarah and Luke personally destroy the bacterial culture, and Sarah uses her contacts to take care of Augur and his exo-suit and to make sure that Augur's victims aren't charged for their crimes. Sarah hopes that the Keratin will now write off Earth as too dangerous to risk invading again -- but if they do ever return, Sarah and her friends will be waiting.