Staines Down Drains S01 E26 The Final Flush

With Dr. Drain's access to Drainland gone, it seems he's defeated for good, but he's no quitter: he finds the Staines' sink-portal (which he created) and uses baked-beans on the purifying-plants, and Gretel, out of spite, unwittingly gives toxic waste to the villains and unleashes them on the city.