Wally, now being age 16, is now legally old enough to be employed. He thus gets a job ahead of summer vacation and any "application rush" that might take place.
His first job is a pretty independent one for a 16-year-old - He is trusted with an Igloo Ice Cream cart. He gets so many ice cream bars to sell a week, and his boss will come by and collect the money for the bars he sold at the end of the week.
But things don't turn out like he planned. Eddie and Lumpy, a little kid who claims his mother will gladly pay him Tuesday, and a girl from Wally's class who employs her feminine wiles and pleads helplessness in the face of a party with no ice cream have Wally extending credit to people who don't deserve it. This leaves Wally having to go around to collect from people who have long since digested his ice cream and see no sense of urgency. He is over three dollars short and the boss is coming to collect. How will this all work out? Watch and find out.
I can't blame Wally too much. Most 16 year old kids in retail are working under the direct supervision of an adult proprietor and have no freedom to extend credit. Beaver has a small but pivotal role in this episode, illustrating the point that family members may not mind calling you a rat, but they don't want outsiders doing so.