"The Old Man and the Lisa" is the twenty-first episode of Season 8.
Mr. Burns loses his vast fortune and turns to Lisa for help in getting it back through environmental means.
Lisa one morning is working on getting recycling as part of a project for school, with the planned result being going to Albany for a field trip. To celebrate the occasion, Principal Skinner has Mr. Burns give a speech on how to become successful at a business. Lisa asks him what he thinks of recycling, although he clearly isn't familiar with it until she defines it. He then expresses disgust at the idea of Mother Nature needing their help, and implies that his wealth is in the two million mark and thus doesn't need to help her, until Lisa corrects him and states its actually closer to the one million mark per his autobiography (which was implied to be ghost-written by Smithers, who admits that not only was she correct about the mark, but his actual monetary wealth is significantly lower than that). Shocked at the implication that he lost money, he abruptly leaves to find a solution.
Mr. Burns is sitting in his office and talking with his lawyers. Unbeknownst to him, the lawyers agree with everything he says in fear of his wrath, going so far as to encourage him while he makes bad investments. It is revealed that Burns has lost all of his money: The bank forecloses on the plant and sells Burns' mansion to pro wrestler Bret Hart.
Meanwhile, Lisa, who is clearly unsympathetic to Mr. Burns' plight, laughs at his misfortune, and Homer reveals that the Nuclear Power Plant, despite Mr. Burns being broke, is not foreclosed, but put under the charge of Lenny Leonard. Lisa and the rest of the school attempt to deliver the recyclables to the Uriah's Heap Recycling Center, although Skinner is outraged to learn that all their hard work only amounted to $.75, which according to Skinner was not even close to the amount of Gasoline he put into the car to make the trip there or the twine to wrap the newspapers in a bundle, let alone for the field trip, causing him to bitterly cancel the field trip to Albany despite Lisa's protests that their work was enough to save one tree (which ironically resulted in Skinner destroying one tree by backing into it before leaving).
Burns lives with Waylon Smithers for a short while, then gets put in the Retirement Castle after he is observed talking to ketchup bottles in the supermarket. Soon, Burns begs for the help of Lisa (whom he had a chance encounter after Homer, who was left severely drunk due to drinking a significant amount of beer cans to allow her to get enough beer cans to fill a bag, suggested she go to the Retirement Castle due to it having "lots of trash" after failing to fill a single bag due to reaching his limit.), who had been pestering him (and the rest of the neighborhood) about recycling. She declines, but after patience and irritation, she gives in. On the condition that he will lose his evil manners she helps him, the two start making money by recycling cans. After a while Mr. Burns has made plenty of money to open the Little Lisa Recycling Plant, all made from recycled materials. Lisa is pleased, believing that Mr. Burns has truly changed, until he shows her 'the best part of all': He is using people's recycled plastic to create a giant net made from six-pack holders, which 'sweeps the sea clean' by hauling all the creatures out of it and selling the meat as animal slurry. Lisa is horrified, proclaiming Mr. Burns hasn't changed his ways, and when he tries to be good he is still more evil.
Everyone but Lisa in town is apparently brainwashed to recycle. Lisa, being comforted at home, receives a visit from Burns, who informs her that he is selling the plant for $120 million. He offers 10% to Lisa, but understanding where the money came from, she turns it down.
Homer has four simultaneous heart attacks (thinking she turned down $12,000 dollars). Lisa later corrects him at the hospital, saying that 10% would actually be $12 million. Homer then suffers another heart attack severe enough to enduce a "Code Blue".
Behind the Laughter
The writers had always thought about having an episode in which Burns lost all of his money and would have to interact with the outside world. Bret Hart, a professional wrestler, makes a cameo as himself.
In the I Can't Believe It's a Bigger and Better Updated Unofficial Simpsons Guide, the episode was called "An odd episode with a not-too-unexpected outcome. The best bits are undoubtedly Burns learning his way around a supermarket and Lisa's realization of what Burns has been up to."