The Decoder is the second episode of the second series of the Sparticle Mystery. It first aired on February 12th 2013.
In the race to the Quantum Nexus to bring the adults back, the Sparticles encounter friends and enemies in their search for the next vital clue.
The Sparticles arrive at the Great British Museum. They decide to head in, but do not close the van door properly. They spot a figure in a cloak with a bird mask on top of the roof of the museum. At first they think it is a mechanical exhibit but it is revealed to be a person. A group of people then arrive on top of the roof and threaten to fire things at the Sparticles unless they leave. Callum tries to bargin with them, but this fails and everyone is pelted with vegetables.
Meanwhile Holly and Serena are spying on the Sparticles getting pelted. Holly then spots the stone head through the open doors of the van while Serena sneaks around the side.
The Sparticles meanwhile are trying to avoid all the fruit and vegetables being pelted at them in a slow motion sequence when an unknown boy comes and picks Tia up and carries her away much to Jeffrey's dismay. Serena meanwhile looks at how Reese has noted down the symbols from the head and rips the page with the symbols from her notebook. Tia meanwhile is talking with her rescuer when Jeffrey comes over. The boy explains that their attackers are the History Tribe and have locked the musuem up to protect the treasures inside. The boy introduces himself as Rocky, the leader of a tribe called the Featherheads.
Rocky brings them to a farm where they raise chickens and breed and eggs and they run it on a generator. It's revealed that they dont have enough Fizzys (the currency of the country) to buy fuel and that they power the ordinary lights on a generator run by children in the tribe. Rocky wants to break into the museum to get the gold to melt it down. The Sparticles (aside from Jeffrey) decide to help the Featherheads while Jeffrey is jealous of Tia and Rocky.
Meanwhile, Holly and Serena manage to find a door but it is locked by a four digit code. She tries various historical dates and gets lucky with 1066, the year of the Battle of Hastings.
Rocky plans with the Sparticles on how to break into the musuem and wants to use the Sparty van, but the Sparticles immediately write this off. Meanwhile, Reese is in the van looking at her notebook and realizes that the page with the code has been ripped out, but starts to rewrite the code. Holly and Serena are meanwhile sneaking around and they spot some disguises and they start to sneak around but are caught by a blue faced person who they distract and run away from.
Jeffrey, meanwhile, is trying to make fuel for the farm using dung. Aa girl stumbles upon him and he explains what he is doing. Holly and Serena are meanwhile in the Musuem's Celtic exhibit and they realize the symbols on the kings neck are Ogham, an ancient Celtic form of writing. Tia is meanwhile exploring Rocky's room. He spots her and admits his mum got him into boxing. Tia and Rocky talk about their parents when the lightbulb starts to flicker and Rocky runs outside to investigate and the Sparticles follow. The camera then shows on a picture of Rocky with someone in his room. In the musuem, the same person is shown under the bird mask when Callum tries to bargin with him and the bird mask boy agrees to let him in. Meanwhile Holly and Serena find the decoder but break a pot causing the people to become suspicious and catch them.
With the children of the Featherheads exhausted, the Sparticles take over. The History Tribe find Holly and Serena and decide to lock Serena and Holly up but Serena manages to get them to give the decoder to them. Jeffrey and the girl meanwhile are trying out his plan - he has put the dung isnside a pot with a lid which he is boiling which will create fuel. Holly and Serena get the decoder, while others try to execute Callum. Back in the van Reese hears something and realizes that a girl is stealing fuel from the Sparty van.
Realising that Rocky is putting his plan to rob the museum into action (albeit with with a tractor to destroy the doors as opposed to the Sparty van), the Sparticles try to follow him with The Sparty van, but it refuses to start thanks to the fuel being stolen so they have to make it on foot. Callum meanwhile stumbles upon a room in the musuem filled with photographs and other personal belongings. The History Tribe Leader finds him and explains that it is a memory room where everyone who goes in leaves behind something of their lost family.
Callum and the leader then see that Rocky has tied the tractor with a rope to the door and yanks them off, prompting the Featherheads to charge in, while Holly and Serena have nearly translated the message on the stone head. Callum and the Histroy Tribe's leader are trying to fight off the crowd, while Holly and Serena finish decoding the message and leave. Back at the farm, Jeffrey and the girl are hiding behind a hay bale as the pot shakes like mad, believing it will explode. Rocky then goes into the memory room and its revealed he History tribe leader is Rocky's brother. Rocky's brother wishes to remember their family, while Rocky would rather move on and face the future, believing his brother is looking at the past. The Featherheads have meanwhile gotten the gold while the Sparticles just arrive. Reese reveals that Serena took the page. Reese retrieves the decoder but a boy takes it off her. Luckily, Reese finds it in the scoop of the tractor, but ends up stuck when Rocky lifts it off the ground. The Sparticles try telling him, but he will not listen. Elsewhere, Jeffrey's experiment produces liquid, but both he and the girl are afraid of the machine exploding, so they draw straws. Jeffrey draws the short straw, but the girl gets it the fuel anyway. The pot then explodes.
Rocky arrives back at the farm while Reese is decoding. She drops the decoder when the tractor goes over a brick and Rocky unknowingly destroys the decoder by rolling over it. Luckily, Reese was able to decode the message and the generator starts up again, now powered by Jeffrey's fuel. The message reads "Behold Artos merciful god who saved us from the Roman sword and lead us to shelter here by the river to the white horse that leaps the highest." Holly and Serena are meanwhile driving away thinking they're ahead of the Sparticles. Jeffrey has figured out the problem with the fuel shortage and the Featherhead tribe return the gold to the musuem and Rocky and his brother make amends and that Rocky can use the solar power if he gives the musuem eggs. The Sparticles split up with Tia going with Callum as there has been a battle somewhere that had a signifigant amount of casualties and the rest of the Sparticles going off while they plan to meet up in three days. Rocky however has stowed away onto the van with the Sparticles.
- Tia and Callum split up from the Sparticles in this episode, but plan to reunite.