It's the Plunger. They set out to find a missing ship, which never showed up in port. Everyone thought it had gone down in a storm, but one of the investors was convinced it had strayed out of the normal shipping lanes and got lost in the Arctic. So they called Frank,

"Do you have a ship that goes in the air and can spot wreckage or rafts?"

"No, that was last week. This week I have a sub. We can look at the bottom of the boat and identify it from there!"

"Uh - we'd prefer an airship, actually -"

"That's not for two more issues."

   
 

The ending of these stories is always as exciting as checking off boxes, but they assure the young reader that all is well.