Three kids go on a canal boat holiday. They spot a rare peregrine falcon flying about and meet a pair of dumb crooks who're trying to nab it. They confide in their friend, who runs a local junk shop, and they all go to investigate. Mr Trim turns out to be the mastermind behind the illegal bird trade in eggs the area, a man called Badger and a woman called Cat Mary. The only threat to their plan is another dodgy-looking bloke, whom the children call The Birdman because they suspect he's the thief. The Birdman is keeping the villains' house under observation as he is actually an RSPB inspector, so Trim and co trick the kids into sending a coded telegram to Trim's buyer. The kids find out what's going on, but when they go to check out Trim's shop Jackie is caught. The Birdman helps the two lads get away, and reveals to the kids he is on the trail of Badger, Mary and Trim. Butch and Trevor find a secret message from Jackie and set off towards London Zoo, where the crooks are planning to ...