When a group of children on a school visit to some caves return to the surface, every single person over 15 has disappeared. Opportunistic gangs roam the streets as Sadiq leads his newly-formed tribe to the safety of the City Hotel. After discovering her mum and sister missing, young scientist Kat takes charge of Liam, the boy next door. They too try to gain entry to the City Hotel, but only Liam gets in, leaving Kat to fend for herself.

Callum has formed a biker gang and has designs on the City Hotel as the gang's new hang out. He befriends Kat as she might be useful to his success. However, Kat is on her own mission to find out what has caused the disappearance of all the adults and encounters Reese at the City Library. Reese insists she can see the adults and feels she has found an ally in Kat. Meanwhile, inside the hotel, Holly is left wondering where her parents are.

Having won her place in Sadiq's gang, Kat leads a team to the University to find food. Holly is taken hostage by what appears to be the only surviving adult. Reese discovers a message from Doomsday Dora, explaining her heart theory - which also explains the curious survival of the adult who is actually - at heart - a child.

The need to generate new sources of fresh food leads Sadiq to free animals from a city farm. This goes very badly until Tia discovers that the cheap perfume she wears is a magnet for the nanny goat.

Doomsday Dora's message has warned the children they must reunite the two parallel worlds quickly or adults and children will remain separated permanently. The ticking clock has started. The children decide to choose who goes on the road to the Sparticle Project and who stays behind to look after Liam and Frankie by an 'Apprentice'-style challenge to source power for the City Hotel.

Jordan, Holly and Ami take being kids way too far, and nearly kill themselves on a runaway waltzer, until Muna, a child caught between the parallel worlds, comes to the rescue.

The lack of grid power has led to the failure of the national water supply. Fresh water becomes the latest challenge for the Ranchers, and Frankie goes native in her attempts to find and save the precious resource.

A postcode given to Reese via Psychic Sarah at the fairground leads the Questers to a simulator where scientists were trained how to run the Sparticle Project. Ami, Jordan and Holly get trapped in the room with the prototype collider's super-cooling systems running, and the temperature is plummeting towards absolute zero.

At the City Hotel, a fox has been at the chickens and the children decide to trap it. This is very tricky, but when they finally track down the animal and are set to destroy it, they decide on a better course of action.

While the team are on their way to the Sparticle Project, Ami is captured in the woods by the Water Rats - a group who love life messing around on the river. The Water Rats are at war with the Menaces - and the Questers, persuaded by Ami, decide to help. Will Ami swap the seriousness of the Quest for a kid's life of fun and games?

Jeffrey is keen to help out more, but his wheelchair needs more power. Tia and Jeffrey discover a satellite communication system which allows them to send a message into the ether, asking anyone out there for the batteries Jeffrey needs. They arrive back at the hotel to a package - it's from Anita at the university and it comes with a message.

Jeffrey is proud of his abundant garden, but when hungry waifs turn up at the hotel, he finds himself at war.

The Questers on their way to the Sparticle Project discover a bucolic village run on communal principals. Reese 'sees' Muna again and with the help of Tamsin, one of the village kids, Kat figures out that the wormhole events are occurring at points where leylines intersect. Some of the Questers, however, have been caught red-handed stealing food and are made to stand trial in the children's court where the issue is whether stealing to survive is justified.

The Ranchers have invited a tribe of seriously idle girls into their hotel. As they take advantage, the Ranchers are forced to fight back - but who will be turfed out of their home?

Holly is anxious when she realises the journey to the Sparticle Project is coming to its conclusion. She leaves her fellow Questers trapped in an underground nuclear bunker, but there is another force at work. A computer who thinks there has been a nuclear war competes against the ingenuity of the children in a battle of wits.

The Questers discover an alarming new threat. Without the adults, and the automatic cooling systems, the world's nuclear power plants are melting down and threaten to destroy the earth. The Questers have to get a message to the adults - but how?

Callum arrives to collect the key to the Sparticle Project from the Ranchers, only to discover them homeless. Tia and Jeffrey don't trust Callum after past encounters. But the key has been lost and sold and Callum is the only hope to getting it back and to reuniting them with their friends.

Ami lapses into a diabetic coma and the Questers rush him to hospital to find some insulin. Switching a generator on in the hospital may just have cost one baby his life. The kids running the hospital need fuel to run the generators to keep the babies alive. Aware of the ticking clock to get to the Project but desperate to put things right, the Questers undertake a dangerous mission as a solution. They have to distract a German shepherd dog who is guarding a yard full of petrol lorries.

Meanwhile, it is Tia's fifteenth birthday. Will she be transported to join the adults in a parallel dimension?

The Questers and Ranchers are reunited at Black Tor Mine where the adults were separated from the children by the machine of the Sparticle Project. But another tribe - the Mystic Moles - the children of the scientists working there, won't let anyone through the doors. Tia and Jeffery share a kiss. Callum arrives with the dictator and Kat kisses him as a thanks.

The Sparticles face a series of challenges before they get to their greatest obstacle. Although they are successful reuniting the worlds, before the reunification stabilises, Kat stops the beam to save Reese's life. This disrupts the realignment process and the two world split again. Doomsday Dora tells the Questers to travel to the Quantum Nexus (hinting a second series).

Just before Kat's dad disappears, he gives her a code to shut down the world's nuclear power plants.