At recess one day, sisters Elsie and Theo spy a nut wiggling on the ground. Out pops Benny--the world's smallest and most gentlemanly walrus. After the girls learn that Benny misses his home in the sea, they send him sailing in a milk-carton boat, along with a trusty band of adventure-seeking slugs. Together, Benny's Brigade (as they call themselves) begin their voyage to a truck-sized island paradise, avoiding the salt water as much as they can. Slugs don't like salt.