There was once a great war, and when it ended, many soldiers were discharged. Among them was Brother Lustig, who received only a small loaf of bread and four kreuzers. As he departed, he encountered a poor beggar (who was St. Peter in disguise) and begged for alms. Though poor himself, Brother Lustig shared his bread and money with the beggar. St. Peter, testing his generosity, appeared twice more in different forms, and each time Brother Lustig gave away a quarter of his loaf and a kreuzer.
With his last quarter of bread and kreuzer, Brother Lustig ate at an inn. Soon after, he met St. Peter, now disguised as a fellow discharged soldier. St. Peter proposed they travel together, using his knowledge of medicine to earn a living, promising to share any earnings.
They came upon a peasant's house where the husband was deathly ill. St. Peter healed the man instantly with a salve. The grateful couple offered rewards, but St. Peter refused. Brother Lustig, nudging him, insisted they accept something. The wife finally offered a lamb, which St. Peter agreed to take only if Brother Lustig carried it.
In the woods, hungry and tired of carrying the lamb, Brother Lustig suggested they cook and eat it. St. Peter agreed but said he would not cook and that Brother Lustig must wait for him to return before eating. Brother Lustig cooked the lamb and, unable to resist, ate the heart—considered the best part. When St. Peter returned and asked for the heart, Brother Lustig claimed lambs have no hearts. St. Peter, unconvinced but letting it pass, said Brother Lustig could keep all the meat.
Later, St. Peter created a deep stream they had to cross. He went first, with water only to his knees. When Brother Lustig tried, the water rose to his throat. St. Peter offered help only if he confessed to eating the heart, which Brother Lustig denied twice. Finally, St. Peter saved him anyway.
They heard of a dying princess. Brother Lustig saw a chance for reward, but St. Peter walked slowly, and they arrived to find her dead. St. Peter claimed he could raise the dead. At the palace, he cut the princess's body apart, boiled the pieces, reassembled the bones, and resurrected her. The overjoyed king offered any reward, but St. Peter refused. Brother Lustig, exasperated, whispered for him to accept something. The king, noticing, filled Brother Lustig's knapsack with gold.
In a forest, St. Peter proposed dividing the gold into three shares: for himself, Brother Lustig, and "the one who ate the lamb's heart." Brother Lustig immediately claimed the heart and took all the gold. St. Peter questioned how that was possible if a lamb had no heart. Brother Lustig reversed his earlier claim, insisting lambs do have hearts. St. Peter, seeing through the deceit, left him.
Brother Lustig squandered his gold and soon was poor again. Hearing of another dead princess, he attempted to resurrect her for reward, mimicking St. Peter's actions but failing to arrange the bones correctly. St. Peter appeared, corrected his work, resurrected the princess, and forbade him from taking any payment. However, Brother Lustig cleverly hinted until the king filled his knapsack with gold.
Outside, St. Peter confronted him for disobeying. As a final act and to keep him from further mischief, St. Peter enchanted Brother Lustig's knapsack to contain anything he wished, then departed forever.
Brother Lustig wasted his money again. Down to his last coins, he used the magic knapsack to steal two roast geese from an inn. He ate one and gave the other to two journeymen, who were then wrongly accused of theft and beaten by the innkeeper.
He later came to a magnificent castle said to be haunted by nine devils; no one who stayed survived. Brother Lustig bravely spent the night there. The devils appeared and tormented him until he wished all nine into his knapsack, trapping them. The next day, the grateful castle owner rewarded him richly. Brother Lustig then took the knapsack to a smithy and had the devils beaten on an anvil, killing eight; one escaped back to Hell.
In his old age, seeking heaven, Brother Lustig consulted a hermit who described two paths: a narrow, rough road to heaven and a broad, pleasant one to hell. Brother Lustig chose the pleasant road to hell. At hell's gate, the doorkeeper—the very devil who had escaped his knapsack—recognized him and, terrified, refused him entry, fearing he would trap all of hell in his knapsack again.
Turned away from hell, Brother Lustig went to heaven's gate, where St. Peter was the keeper. St. Peter refused him entry. Brother Lustig then asked St. Peter to take back the enchanted knapsack. When St. Peter took it inside the gates, Brother Lustig wished himself into the knapsack, thus cleverly entering heaven, and St. Peter was forced to let him stay.