With Christmas coming, Del's application to buy the Trotter family's council flat in "Yuppy Love" has finally been granted, albeit rather late and suddenly, meaning he now owns the flat but is not as able to pay the mortgage as he was originally planning to. He also inherits Grandad's old allotment and receives a summons from the council, ordering him to remove a public health hazard from his land. The health hazard turns out to be barrels containing an unknown yellow substance. Del enlists Trigger and Denzil to help him get rid of the yellow liquid, saying he will help them though he wears a diving suit, by transporting it in Denzil's van to the "24 hour" council waste disposal depot (which is closed at night), although they ultimately dump it "in a pond."
Del accompanies Rodney to an organic health food shop owned by a wealthy gardening enthusiast named Myles. When he sees the high price of the store's produce, Del develops an interest in gardening and invites Myles to the allotment to advise him on growing vegetables. At the allotment, Myles notices a mysterious water source - which Del calls the Peckham Spring - and suggests that if the water is pure, it could be bottled and sold. However, unsighted by Myles, Rodney notices that the "spring water" is actually coming from a nearby tap connected to a hose and concealed by rocks. Del instructs Albert to take a sample of the water for laboratory testing but, being part of the scam, Albert uses bottled mineral water instead. The water receives a certificate of purity from Myles' committee, giving Del permission to sell it.
A production line is set up in the Trotters' flat, with Del filling up bottles from his kitchen tap. Peckham Spring Water quickly becomes one of Myles' biggest-selling products, and is given a trial at a major national supermarket. Sales are further boosted by the drought warnings issued by the local water board, who believe there is a serious underground leak due to the high volume of water being used by Del. The Trotters earn enough money for a Christmas holiday at the Grand Hotel, Brighton. In the final scene, as the Trotter Family go to sleep in their hotel rooms, the BBC News reports that a local reservoir has been contaminated by the same yellow liquid dumped by Del, Denzil, and Trigger. As a blissfully unaware Del switches off the light, a bottle of Peckham Spring on his bedside table glows yellow.