"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.
Mr. Burns is sitting in his office and talking with his lawyers. Unbeknownst to him, his 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.
He lives with Waylon Smithers for a short while, then gets put in the Retirement Castle. Soon, Burns begs for the help of Lisa, 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 Burns has made plenty of money to open the Little Lisa Recycling Plant, all made from recycled materials. Lisa is pleased, believing that Burns has truly changed, until Burns show her 'the best part': 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.
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). She later corrects him at the hospital, saying that 10% would actually be $12 million. Homer then suffers another heart attack.
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."