Sarah Jane volunteers to chaperone Luke's school trip to St Margaret's House Museum, a Victorian recreation an hour's drive out of London. She only realises after the trip has set off that her presence is embarrassing her adopted son as he tries to fit in with the other children. She thus restrains herself from correcting the museum's historical inaccuracies based on personal experience, and spends the ride back to London chatting with the history teacher, Mr Bradbury; his earnest interest in history bores her to death, but she does not tell him that she's visited medieval England in person. On the way back, the coach stops at the Stone Whisperers, a stone circle now enclosed in a state-of-the-art visitors' centre thanks to a lottery grant. The curator, Dr Riley, is just as dull as Mr Bradbury, and the children quickly scatter to explore the interactive exhibits. Sarah joins Luke, Maria, and Clyde as they examine the circle, which appears incomplete, as though some of the stones have been removed. Luke is strangely drawn to the King Stone, a larger stone set some distance away from the others and glowing slightly with a natural phosphorescence.

Professor Jacqueline Lauton notices the children's interest and tells them that she's about to finish analysing the stones using ground-penetrating radar; she hopes to find out what they're composed of and what makes the King Stone glow. Legend says that the stones are the remains of twelve warriors who defeated a tyrant king in battle, imprisoned him within the King Stone, and turned to stone themselves in order to keep him trapped. Lauton is more interested in the reality of the stones than in the myths surrounding them, but Luke points out that the myth teaches them something important about the apparently incomplete circle. The King Stone is also known as the Thirteenth Stone, and since there are only twelve stones in the arc, that means that it's complete as it is and no stones have been removed from it after all.

Lauton invites Sarah and the children into the computer suite to see the results of her scan for themselves, and Sarah realises that Lauton feels unappreciated here and wants someone to share in the wonder of her discovery. Given that the equipment cost two million quid, however, Lauton is also concerned about what might happen if her experiment fails. She has set up the monitors in the same positions as the stones in the arc, and because Clyde promptly seats himself in the chair before the monitor representing the King Stone, he's the first to realise what the pattern represents. Each of the other twelve monitors is facing him, which means that each of the Sentry Stones is facing the King Stone. It's not a semi-circle, it's a parabola, with the King Stone at the focal point.

Mr Bradbury calls Sarah and her friends to return to the coach, but they tarry in the lab, watching as the computers complete their scan and begin to map out the interior of each Sentry Stone. The 3-D models are colour-coded, indicating the presence of minerals such as sandstone, granite, and quartz... but in the centre of each stone is a solid black space of unidentifiable material, which resembles the shape of a crouching soldier. And the black space in the centre of the Thirteenth Stone is textured with light greys and browns that look like the folds of a cloak draped over a standing figure... which appears to be screaming. As the scan completes, the shadowy face in the King Stone turns to look directly at Luke, who collapses to his knees as all of the lights in the lab go out simultaneously. Sarah pulls Luke back to his feet, but the red emergency lighting is casting a shadow across his face -- and the shadow looks exactly like the face of the King, except that now it's laughing.

Lauton, trying to convince herself that this is just a power surge, stays in the lab to recover her data from the stricken computers while Sarah, Maria, and Clyde support the dazed Luke outside. The other children are waiting by the coach with Bradbury and Riley, but as Sarah and her friends approach the facility's doors, the shadow on Luke's face deepens and the doors slam shut by themselves, trapping them inside. Sarah's sonic lipstick fails to open the doors, and Maria finds that she can't get a signal on her mobile. Dark clouds cover the sun, leaving the stone circle illuminated only by the red emergency lighting. In the weird darkness, it's clear that their shadows are pointing directly at the King Stone -- until the Sentry Stones begin to split open, revealing warriors with glowing red eyes wearing glistening black armour, just like the shapes on the computer monitors.

The shaken Sarah says aloud that they should have put two and two together, and Luke immediately replies "four." Clyde tells Sarah that Luke sometimes switches off mentally when classes get too boring, and only seems to come back to himself when he has an interesting problem to solve. Sarah and Maria thus start to bombard Luke with maths questions, and as he answers them, he seems to regain control of himself. But the shadow over his face refuses to fade, and when the stone soldiers lift their swords and advance on them, the shadow speaks through Luke, telling Sarah to get him to safety if she wants Luke to live. Sarah and her friends carry Luke into a nearby lecture hall, barricade the door, and then slip out through the other entrance; fortunately, the Sentries are very methodical, and they all smash down the door and enter the hall rather than circling around to block the other exit.

Despite the distraction of the maths problems, the shadow over Luke's face is getting darker. Sarah and her friends take Luke to the King Stone in the hope of learning more, and see that the phosphorescence has brightened into a fierce orange glow. At the heart of the Stone is a dark shape with the face of the King. The shadow speaks through Luke, identifying itself as Ravage, the self-proclaimed greatest warrior that the planet Amital had ever known; he seized the throne through force of arms, but was eventually defeated by the forces of the Republic and bound in a psychomolecular prison. The Sentries sacrificed their own life force to keep Ravage contained, but their power waned over the centuries until Lauton's scan inadvertently transferred the remaining life force out of them and into the King, giving him the power to transfer his mind into another body. Unfortunately, Luke just happened to be present at the time, and his mind is still clear and fresh, a blank slate that Ravage can possess without being overwhelmed by his victim's lifetime of experience. Soon he will crush Luke out of existence and occupy his body -- and rather than let that happen, the warriors are willing to kill Luke before the transfer is complete.

One of the Sentries finds the group standing by the King Stone, and Clyde tries to fight it off while the others escape. Sarah Jane and Maria urge Luke to keep fighting Ravage, but Ravage has committed himself fully to this escape attempt and is fighting just as hard to hold onto his new body. Sarah blames herself for not letting Luke grow up enough to fight Ravage on his own, and she is forced to let go of Luke and tell him that only he can save himself now. Maria begs the advancing Sentries to give Luke a chance, and they pause and watch as Luke struggles with Ravage for control of his mind. Ravage desperately insists that Luke is still young and unformed, and that he doesn't know who he is yet -- but Luke is stronger than Ravage believed, and he knows that he has friends, a mother who loves him, and a life of his own. With a great effort, Luke casts Ravage out of his mind, and the King Stone stops glowing as the shadow disappears from Luke's face.