One afternoon at Park Vale school a boy named Jacob is sitting in detention, under the watchful eye of Mr Cunningham, who is concerned he has uncharacteristically been involved in a fight. Soon the other students are taunting the young boy, who appears to have a strange alien symbol marked on his hand.

Rani enters the detention room, where the silenced Mr Cunningham and the other students ask mutely for help. Running out, she hurries to the bathroom, where Jacob has removed a glowing blue pendant from around his neck. He orders it to stop what is going on and immediately Mr Cunningham and his pupils are freed. They all run from the building out of fear.

Having thrown the pendant to the floor, Jacob opens the bathroom door to find Rani waiting for him. As she enters, hearing a strange noise coming from within, he hurries away, leaving her to find the pendent and pick it up. Suddenly her phone rings, and after agreeing to meet her father in the car park, she pockets the pendant and leaves, unaware Jacob, whose face has returned to normal, has actually been watching her.

Some time later, Sarah drives Luke to Clyde’s house, where he will be staying for the weekend whilst she investigates a matter at Tarminster Hospital. Clyde’s mother Carla is happy to look after the boys, and Sarah thanks her before leaving for the hospital. Carla follows the boys inside the house, where she reveals to Luke that she has been teaching Clyde to cook, which he has found a secret talent for.

Across town Rani and her father are discussing food as well, only they have decided to order takeaway instead. Initially they argue over what food to have when suddenly Haresh begins agreeing with everything Rani says – just like Jacob and the detention room. Rani realises this is because of the pendant and starts asking her father to perform various humiliating tasks, before claiming nonchalantly that her father would die if he could see himself. He takes this literally, and asks Rani if she wishes him to kill himself.

Hurriedly Rani backtracks, and orders her father to forget what has just happened. She places the pendant back in her pocket then looks at her hand – she has an identical mark on her palm to Jacob.

The door bell rings and she answers it, finding Jacob waiting for her. He begins talking about the mark, which she finds is fading. Jacob explains that this is due to her only having used the pendant once. He tells her he found the device at the building site at school, and claims it has addictive properties, admitting that when he discovered he could use it to order people to his will; he found it difficult to stop. The more the pendant is used the worse the mark becomes. Rani claims she knows someone who can help, and hurries alone to Sarah Jane’s attic, where she discovers a note ordering the kids to do nothing if they uncover any alien activity.

Ignoring this warning, Rani tries to activate Mr Smith, but discovers Sarah has switched him off. She decides not to go any further and leaves the pendant hanging on a nail sticking out of a nearby wall. Realising the ensuing weekend will be a quiet one without Sarah around, she leaves.

That night, as Clyde and Luke lie in bed, Luke looks at an array of pictures stuck on Clyde’s wall, and discovers his friend has another secret talent – he is quite the artist. Conversation briefly turns to Clyde’s father, who no longer lives at home, but Clyde brushes off the topic, claiming he has his mother, and that’s all he needs to get by.

The next morning a taxi pulls up outside and the doorbell rings. Clyde awakens and runs to answer it – it is his father.

Back in Bannerman Road, Rani is relieved that the mark on her hand has not reappeared, and as a sort of celebration she makes her father breakfast. She explains that she is planning to meet Luke and Clyde later that day, unaware of the events unfolding in Clyde’s house.

Clyde’s father, Paul, explains that he is now living in Germany, and claims he has returned to see his son. However, his return is not a welcome one. Clyde reveals that years ago, when his father ran out on them, he blamed his mother for driving him away. His life began to take a turn for the worse, and he ended up being expelled from school.

Paul insists he now wants to make amends, but Carla is not happy. However, Clyde claims he does want to walk to his father, and agrees to spend some time with him, leaving Luke with his mother for the day.

In Sarah’s attic the pendant begins to glow, and Rani finds herself thinking about the device obsessively. Deciding to find out more about the object she leaves her home (under the false pretence of needing to water Sarah’s plants) and calls Luke, whom she tells to hurry to the school field.

In a local park, Paul and Clyde sit on the swings. Paul finds it hard to connect with his son, but still he makes the effort. However, Clyde is more interested in why his father left, and Paul reveals that when he and Carla fell out, he decided he had to go. Clyde, on the other hand, knows the truth – Paul ran off with his auntie Mel. Clyde is still angry at this, but Paul reveals he never forgot about his son – he carries around a picture of him in his wallet.

Back at Clyde’s house, Luke prepares to leave, and says goodbye to Carla. They talk about the fact that Luke has no father either, and she asks Luke to remember something – if his father ever turns up, as Clyde’s has done, he must never forget his mother.

In the park, Clyde asks his father to teach him some German, which he does before asking his son what he does in his life. Clyde toys with the idea of telling him about life with Sarah Jane, and after Paul tells him they shouldn’t keep secrets, Clyde tells him:

“Dad…I save the world.”

Meanwhile, Luke and Rani search the building site at the school, hoping to find some clue as to what the pendant really is. Finding nothing, Luke decides to return home to see if Mr Smith will open for him, and Rani follows.

Clyde explains to his father that he and his friends have saved the world several times, but finds Paul is somewhat sceptical. He decides to prove what he says, and takes him to Bannerman Road, where Clyde gains entry with the spare key. They go to the attic, where Paul examines the various objects lying around on Sarah’s desk.

Clyde picks up one of the devices – a holographic postcard reader – and shows Paul a message he and Luke have recoded for Maria, who is now living in America. Paul is astonished by the alien technology and finally acknowledges what his son has achieved – he is proud. However, Clyde begins to realise his dad is only interested in exploiting this technology for money, and chides him. He then begins explaining about Mr Smith, and is unaware that Paul has been examining the alien pendant, which seems to be calling to him. He pockets the device without his son noticing, then the two hurry downstairs.

On Sarah’s front drive they are met by Haresh, who is concerned Clyde appears to be trespassing. Clyde claims he is watering the plants, unaware Rani has also used this excuse. Haresh pursues the inquiry but upon Paul’s command of “drop it”, he drops the empty pizza boxes he has been carrying. Commenting upon the takeaway reciprocals Paul claims Haresh should exercise, and immediately he begins jogging on the spot.

Taking to the streets of London, Clyde asks Paul about the pendant, which he tells him not to worry about. Sure enough, Clyde does as he is told, and further agrees to spend time away with his dad, without worrying about his mother.

In the attic, Rani and Luke debate what to do, worried that if they call Sarah she will be mad with them for letting Paul see the attic. Nonetheless they decide they must phone her. However, her mobile phone is switched off – she is in the hospital hunting an alien slug creature.

Luke then has an idea, and uses Sarah’s ordinary computer to open up a webcam discussion with Maria, who is with her father in Washington D.C. Luke explains that he needs to speak with Alan, and sends across a drawing of the pendant Rani has made. Luke asks him to search the UNIT web archives for any traces of the device, hoping to find out what its purpose is.

Back in the city centre, Paul and Clyde go to a car dealership, where Paul uses the power of the pendant to secure a large red sports car for free. Clyde then receives a call from Luke, who tries unsuccessfully to convince him that they are friends. Clyde ignores him and cancels the call, leaving Luke somewhat upset. Rani enters, explaining that she has checked on her father outside – he will not stop doing press-ups.

Whilst Clyde and Paul take to the roads – the latter beginning to feel the effects of the pendant upon his arm, Alan manages to find a file about the pendant. Inside is a picture, detailing what happens when the pendant is used. The photo shows a man covered in marks – spreading across his face. His eyes have also changed, just like Jacob’s when he began transforming back in the school bathroom.

Maria explains that the pendant belonged to a warrior race called the Berserkers, who used the devices to swell the numbers of their armies – they turned ordinary humans into Berserkers. The one found at the school was last seen in the 1940s, at an army barracks that was built on the ground where the school now stands. Maria realises that if Paul is using the pendant then he – and Clyde – will be in danger.

Luke promises Maria he and Rani will deal with the problem but Alan has another trick to help them. Using a UNIT tracking system and Clyde’s mobile phone number, he traces them as they go about the city, bagging free things with the use of the pendant, including a leather jacket for Clyde. All the while Paul fights off the control of the Berserker, catching glimpses in mirrors and other reflections the image of the creature he is soon to become.

Luke and Rani hurry to Clyde’s house, where they tell Carla they need her help. Explaining that Clyde may be in danger, she agrees to help them trace him and the three of them pile in Carla’s car.

On the road, Paul sees that Clyde is upset, and orders him to reveal what is bothering him. Stopping by the side of the road, Clyde explains that he is still upset about being left alone. He claims he used to fear being left by his mother as well, but Paul makes him forget all of those feelings, and then tells him that he intends to show Clyde the world, however first they must find a boat. Again Clyde has his doubts – worried about what Carla will say. Once again Paul diffuses the situation with the pendant, this time making his son forget all about his mother.

Also on the road, Carla tries to phone her son but there is no answer to her call. As the marks on Paul’s arm begin to get worse, Luke manages to augment Carla’s satellite navigation device to show the UNIT tracking system – now programmed to follow Clyde. Over the phone, Alan urges Rani and Luke to speak to Sarah as soon as possible – worried that when Paul transforms fully he will be too dangerous for them to handle alone. He signs off and he and Maria are left to wait; only Maria has now formulated a plan to help her friends…

Paul continues to fight off the effects of the pendant, and soon he and Clyde arrive at a Marina. Close on their tails, Carla soon realises what Paul is intending to do and puts her foot down to get there faster.

At the Marina itself, on one of the jetties, Paul’s transformation gets worse, and the mark spreads across his face, causing him to keel over. Soon Carla arrives with Luke and Rani, who are worried about Clyde’s mother finding out about the pendant. Sure enough, when Carla rushes over to a worried Clyde, whom she discovers no longer remembers her – she sees that Paul is no longer human – his eyes have changed.

Rani and Luke confront him and Rani explains about the effect of the pendant. They try to get Clyde away from his possessed father, but Paul grabs him and orders the others to stay back – threatening to make them walk into the water and drown themselves if they do not obey. He turns to Clyde and claims that he has become a Berserker, and his son will follow his orders.

“Oh, I don’t think so.”

The whole group turns to see Sarah Jane has arrived – standing some distance away across the Marina. Clyde breaks free of Paul’s grasp and runs to her, still recognising her amidst the crowd of strangers confronting him. Sarah tells him to trust the others and walks with Clyde to confront Paul, who is now convinced he is a soldier, ready to fight the war of the Berserkers.

Sarah tells Clyde that he must remind Paul who he really is, and the young man walks up to his father, urging him to recall the brief time they spent together before he left to go to Germany. He then recalls the events of the afternoon – and soon Paul flinches, on the verge of remembering. Carla joins in, asking Paul to remember the day they met and fell in love. Once more he flinches, and again at the memory of returning for his son that day.

Sarah holds out a mirror for him to see his new form, urging him to recall his life before he was taken over by the Berserker. Suddenly he begins writhing in agony. He falls to the floor as the marks detract from his body – retreating to his hand before disappearing completely. At last he is free, and he throws the pendant to the floor, reversing all the orders he has made – thus freeing Haresh, who has all the while been exercising on Sarah’s driveway.

Clyde is also healed – remembering his friends and his mother once again, much to their joy. Clyde then turns to the recovering Paul, and Sarah realises the family must be left alone. Taking Luke and Rani back to her car she tells them Alan and Maria contacted her and told her what had happened, thus alerting her to the danger Clyde and Paul were in.

Clyde, meanwhile, picks up the pendant and asks Sarah what to do with it – she insists he already knows. He holds it in his hand and contemplates that he could use it to make his father and mother fall in love again. Paul claims that if he does history will simply repeat itself and the two will part again. Clyde says he could stop this happening but both his parents know that their relationship wouldn’t be real.

Carla goes back to her car, and Paul and Clyde are left alone. Paul reveals that his new partner Mel is pregnant – it is this that made him flee back to London, he was scared of committing himself again. Realising at last that he doesn’t need his father, Clyde tells him that the new baby will, and begs him not to mess up another life by running away again. The two embrace, then Clyde hands back his leather jacket, telling his dad to return everything he has taken.

As Paul drives away, Carla rejoins her son by the waterside, knowing at last what her son does in his alter ego life. She confesses that she cannot comprehend what has happened, and will not be able to cope knowing the danger he is in. To save her worry Clyde uses the pendant to make her forget. With her mind altered, Clyde tosses the pendant into the water.

That night, as Sarah walks about the attic, Clyde enters. They both sit on the sofa, and Clyde apologises for letting Paul into the attic. Sarah forgives him, and he asks her about her parents, whom she claims died when she was just a baby. She confesses that sometimes she would do anything just to see them again, but for now she is more interested in how Clyde is feeling.