The spy hides the sonar in the pocket of one of the underwater manikin's jacket midway though Bud's tour. Flipper is onto the spy ... as always. First he nose-butts him in the stomach preventing him from retrieving the hidden item he stole and then he guides Bud back to the museum right at the time the crook returns for the second removal attempt. A brief underwater struggle ensues between him and Bud but Flipper is ever present to protect.
Porter and Sandy return to the site where Bud's museum begins after alerting the local police just in time for the crook to surface with the stolen sonar in hand and is in custody after a brief struggle with Porter.
There had to be something mischievous that Bud did and sure enough, he put one of Porter's sports jackets on one of his underwater characters without permission. Said coat shrinks in the salt water so its almost the right size for Bud to wear and for his trouble Porter 'fines' him the entire takings from the museum: $9.50 which in 1965 is a pretty good income for an afternoon of diving with kids (2014 equivalent would be $75!).