After accidentally interrupting a garden party at his neighbor Mr. Wilson's house, young Dennis Mitchell witnesses the villainous Switchblade Sam stealing a valuable coin collection as sees it as his duty to get the coins back to their rightful owner. Dennis encounters many adversaries who would unwittingly deter him from capturing the thief -- not the least of whom is his overzealous coach in the school gym -- but armed with his water pistol, slingshot, or peashooter, the young hero must make his way through to the end.