All Westwijk, a Dutch coastal holiday resort island, is excited about the arrival of cheerful celebrity singer Bo Monti and his haughty manager Franck Fellani. Recently moved-in Jack Appelboom, his sister Tessa and their local friend Chris Loman find a stray German shepherd dog and want to adopt him as fourth gang member. Mayor's daughter Chris braves her posh, airhead mother to keep the dog after it's discovered in the storage shed.

Everyone on the island wants tickets for Bo Monti's concert. Jack's gang, like many fans, passes the night in sleeping bags to beat the cue, but oversleep. Jack's mate, surf shop keeper Charlie, knows someone, but could only get hold of a single ticket, so the trio's last hope seems being in the mayor's party on the guest list. A nasty reporter is cared by Koesja and convinces Chris's ma the 'beast' must go. So the brat takes revenge by cuing her clueless mother as if the Duch colonial age classic she must pretend having read to join the push book club were SciFi.

The tickets are ruined because Chris fails to mind them or playful dog Koesja. So the kids each try to get in any way the can, the girls even try try dirty tricks. Jack meanwhile is delighted that his favorite rock band also arrives on the island and auditions guitarists, so he he tries out. When Bo Monti finally presents the singer on stage, it's only to find out right there that the celebrity has gone missing.

Now Bo Monti is missing, the trio is determined to investigate, even more after the mayor is 'called to account' by the press due to mean reporter Ciska's grudge against the kids on account of dog Koesja. To pay for the damage caused by Koesja because she never minds him properly, Chris must take a beach job which Jack arranges in his friend Charlie's surf shop, but she messes it up and eve abuses clients. Jack uses his band connection in the theater to get the trio in Bo's dressing room, where they even find cross-dressing stuff, and to see the surveillance video

While the jealous girls bicker endlessly, clever Jack finds a way out when they get locked in and leads the trace of a the mystery woman in blue's blouse from Bo's room to a hidden caravan. Chris's failure to mind dog Koesja cost a vase, and nobody is dumb enough to (re)hire the rude imp for any job. Mean newspaper reporter Ciska Beerenpoot starts a campaign to exploit the case and suggest it's the mayor's responsibility.

Thanks to clever Jack and Koesje's nose, the kids find a piece of the blue clothing of the woman seen sneaking out of Bo's dressing room in there and follow it. Ciska plans a press conference to spread her theory that the mayor is responsible for the singer's alleged abduction, but the kids now look forward to making her look stupid. Chris's failure to mind Koesja ruined her mother's new Ming vase, so she promises again to pay it off with a job, but only Jack's mate Charlie takes her in the surf shop and soon regrets it more then he expected

A recorded ransom demand confirms that Bo Monti has been kidnapped, although he himself booked a flight abroad. The kids mistrust singer Carlo Korenmolen, he eagerly seizes the opportunity to perform in Bo's place.

Jack is furious when Chris' inability to watch Koesjoe wrecks his guitar, which he needs for the formal band audition, but for once she can solve it by offering him the more expensive one she stopped using long ago. Mean Ciska steals the kids' idea to appeal for donations to a ransom fund. Lurking in the dunes near where the blue lady disappeared, the kids, who suspect the ransom demand doctored, stay despite the singer's manager's attempt to chase them and find a man with Bo Monti's voice, who claims to be his brother Jan Spijker.

All Westwijk, a Dutch coastal holiday resort island, is excited about the arrival of cheerful celebrity singer Bo Monti and his haughty manager Franck Fellani.

All Westwijk, a Dutch coastal holiday resort island, is excited about the arrival of cheerful celebrity singer Bo Monti and his haughty manager Franck Fellani.