Do you really need a mobile phone to be "in"? Jojo is convinced of it. But then it turns out that a tricky case like this is solved mainly with brains, a good friend, and a brilliant Jo-Jo trick. After an argument with his parents, who refuse to buy him – the private detective! – a mobile phone, Jojo escapes to his gang's secret hideout. There, an exciting case begins to unfold: Jojo overhears a few teenagers and finds a mobile phone that he grabs without thinking. Shortly after, it turns out that the phone is stolen, and Jojo is at a loss about what to do. He wants to return the phone to its owner, but doesn’t that make him a thief? Luckily, Jojo has Jannis. The two detectives come up with an almost perfect solution to their problem and are not afraid to stay on the trail of the teenage phone thieves.