Eddie and Hodge have cleaned the sludge out of the chugwash. Now they're going to refill the chugwash soap, then clean the underground tunnels using the Scrub-A-Chug - a fantastic piece of rolling stock with brushes that cleans the ceiling of the tunnel. Their work is interrupted when they get called away on an emergency. Wilson decides to be helpful by finishing their chores. Unfortunately, he accidentally replaces the chugwash soap with chugwash sludge and when he tries to clean the tunnels with the Scrub-A-Chug, he ends up making an even bigger mess! Happily he figures out a way to use the chugwash soap with the Scrub-A-Chug and when Eddie sees the sparkling clean tunnel, he's thrilled. How clever of Wilson to discover that chugwash soap actually helps the Scrub-A-Chug to clean even better!