Felix (13) spends the summer break with his best friends Sammy (13) and Noor (12) at their secret hangout, a hidden swimming pond in the woods. Their quiet holiday is over when Felix makes an amazing discovery: his vintage walkman picks up sounds from the year 1989. The three friends are fascinated by the weird, funny fragments from this long-gone era. Until they hear something disturbing. In 1989..

Felix and his friends use the walkman to find Sammy's cousin Robin, who left in 2012 for mysterious reasons. The adventure brings them to the Morgenster residence, where they meet Sera, who rents a studio there with her sister. They turn out to be victims of Marco Claes, a shrewd landlord and also Robin's former employer. It soon becomes clear that Marco has a dark secret... and that he is..

In season 3 of 3Hz, the Walkman tunes into the future! Felix and his friends discover that in 2043, all the inhabitants of Roegerhout will be dispossessed and their entire village, including the swimming well, will disappear. All to make way for a brand new, green power plant. With the help of the walkman, the friends find out that the power plant will not be built on top of Roegerhout by chance,..