The episode starts with a thirteen year old girl, Maria Jackson, and her recently divorced father Alan moving into a house opposite journalist and former time-traveller Sarah Jane Smith. The night after they have moved in, Maria is woken by an ethereal light emanating from Sarah Jane's house, which she discovers, to her amazement and later horror, to be Sarah conversing with a star poet,or arcateenian.
The next morning, her neighbour Kelsey Hooper visits and welcomes her. She invites Maria into town, using the free "Bubble Shock!" bus to travel there and tour the factory. Once they arrive at the factory, they are lead to a security scanner, which surreptitiously collects their DNA to transfer to an "Archetype" under the supervision of the factory's owner, Mrs Wormwood.
Sarah Jane, having overheard the girls making plans, follows them to the factory and interviews Mrs Wormwood, and how she was able to get approval for the drink so fast, and why the Bane, an ingredient unique to the drink, was "resisting" analysis, to which she is told that all that Bubble Shock! is doing is satisfying the needs of the Western world. On Sarah Jane's way out, she is almost killed by Wormwood's secretary.
Kelsey wanders from the tour and attempts to phone her friend, but disturbs an unknown beast, the Bane Mother, to the annoyance of the staff, to which Wormwood orders the alarms switched off and Sarah Jane killed. Maria attempts to phone Kelsey, but sets off the alarms again, causing the Archetype to escape, transferring the focus upon him. Maria, while escaping, encounters him, who just mimics her, and they escape into a women's bathroom. Sarah Jane then enters, and although they are surprised to see each other, they manage to escape the factory, albeit without Kelsey. When they arrive back on Bannerman Road, Sarah warns Maria not to get involved as her life is too dangerous.
Meanwhile, Kelsey is accosted by the factory guards, which causes her to rant about the treatment she has received. Wormwood subdues her by revealing her true form, and having discovered Kelsey and Sarah Jane live on the same street, uses her PR representative Davey to escort her home. Once there, Maria realises upon sight of Kelsey that Davey had discovered Sarah and tries to run into Sarah Jane's house. Eventually realising their intent, Sarah Jane brings them inside, with Davey, now as a Bane, giving chase. Sarah Jane is able to repel Davey and then, upon discovery of the attic by Kelsey, tells the teenagers about aliens: years ago, she met the Doctor, a man like no other, who took her through time and space. The adventures suddenly ended, but when they met by chance (in "School Reunion") not long ago, they realised they were still fond for each other.
Once Sarah Jane has analysed that the Bane ingredient was sentient, she contacts Wormwood and politely requests that she leaves Earth. Wormwood refuses, and in retaliation takes control of the majority of the human race. Sarah Jane races to the factory, but cannot enter until she drives the bus into a wall. Wormwood reveals the Bane Mother, and explains the Archetype is a conglomerate of human DNA designed to be investigated so that Bubble Shock could be improved. The Archetype uses an alien communicator Sarah Jane had been given, realising that the signal would be strong enough to kill the Bane Mother, and Sarah Jane, Maria, and the Archetype escape as the factory explodes.
The following evening, Sarah Jane agrees to adopt the Archetype and agrees with Maria to call him "Luke", since that was what she wanted to name her child if she ever had one. The episode closes with a monologue by Sarah Jane that while space may be strange, adventures may be had on Earth, if one knows where to look.