"Zantetsuken's Lament" note Also released in English as "Iron Cutter Sword With Blues". Original air date of November 5, 1979.
Inspector Zenigata has finally tracked Lupin down to his hideout, but when he arrives, he finds a note on the door: "Traveling Lupin III". Breaking down the door, he finds Fujiko alone inside, who tells the detective that Lupin, Jigen, and Goemon have gone to a hot springs to relax. Sure enough, the master thief is enjoying the hot springs with lots of sake and his usual lechery. Jigen has taken an even simpler route, sitting in a field, enjoying nature, classical music, and good wine.
Goemon, as always, is training. Meditating on a cliff near the sea and perfecting his cutting skills on bundles of grass, he is watched by Nami, a local high school school girl; watching him as he trains, she teases him about his old-fashioned clothes and ways before departing. Lupin, watching from nearby, observes her flirting, but then tells his friend that she was up to something. He throws rocks at the bundles of grass, revealing the perfectly sliced steel beams Goemon placed inside. However, half of one of the beams is missing.
Nami returns to her home and her elderly grandfather. A swordsmith, he eagerly takes the beam she stole; he wants to use it to help him make a sword better than Goemon's legendary Zantetsuken. Examining the perfectly cut steel closely, he takes a sword he made and attempts to replicate the cut. His blade only makes it halfway through. He wonders what the secret of Zantetsuken is; Nami, watching his efforts, seems determined to help him. That night, Nami goes to visit Goemon, who has already gone to bed. She openly asks to borrow his sword, so her grandfather can examine it. The samurai naturally refuses, but then grabs Nami's arm and tries to pull her into bed with him! She struggles and slaps him to get away, and as she runs, Lupin comes out from under the covers with a wig; he complains he was only teasing.
Meanwhile, the real Goemon is meditating on the ocean cliffs while holding out Zantetsuken in the falling snow. Spying on him, Grandpa finally realizes the something special about the sword; the snow piles up on Goemon's clothes as he sits, but as soon as it touches the blade, it melts. Something about the sword makes it naturally hot. Grandpa asks Nami to see if she can find out the temperature of the blade. She returns to Goemon's room that night. Refusing to turn around, he asks why she is visiting, unaware that she has taken off her clothes. She taunts him by telling him she was in bed with Lupin the night before and he has a cute mole on his thigh. Telling her she is a liar, Goemon turns to threaten her with Zantetsuken, but is surprised when his blade rests against her bare skin. He storms from the room, going to make sure that Lupin hasn't taken advantage of her. Meanwhile, Nami returns to her grandfather, remembering how hot the sword felt against her skin.
Grandpa tests his next attempt against Nami's skin to compare to Zantetsuken. She claims it's the same, so Grandpa makes another attempt to cut the beam. This time, he not only cleaves through the beam, but the anvil below it. Delighted, he tracks down Goemon the next day and challenges him, but Zantetsuken easily makes work of the old man's blade. As he leaves Grandpa in tears, Nami disgustedly asks the samurai if he's satisfied he beat an old man. Goemon remains unmoved.
Grandpa decides to try one more time, but he's fallen ill and is growing weaker. Nami is right by his side, measuring the sword's temperature and finally deciding to help him shape the blade with a hammer of her own. They are unaware that Lupin has been watching them the whole time; as he leaves, he cryptically remarks that Goemon should be prepared to lose now.
The next day, Goemon goes about his exercises as Nami makes one last plea for her grandfather. Goemon firmly tells her that no matter what, he will not let her grandfather win. He cuts another bundle of grass, and is devastated when Zantetsuken simply breaks in half. The grass falls away to reveal Grandpa's sword that Nami had switched in the night before. Grandpa, watching nearby, crows his victory, but suddenly falls over. Nami rushes to his side, where he tells her that he now has no regrets. He dies at peace in his granddaughter's arms.
Goemon tracks down Lupin, and accuses him of having something to do with his broken sword. Lupin confesses that he had switched the sword that Grandpa made for Zantetsuken before Nami planted it. Goemon tries to act angry, but in the end, he has to admit that it was only right the old man die happy. Lupin assures his friend he will trade the swords back soon, and gives his friend something to play with in the meantime: a gag sword that sprouts a flower.
This episode features examples of:
- Absurdly Sharp Blade: Not only Zantetsuken, but Grandpa's attempts at creating a replica. This episode seems to imply that the sharpness of Goemon's blade is more important to his success than his Implausible Fencing Powers.
- Bears Are Bad News: Lupin is surprised to find one has joined him in the hot spring instead of the pretty lady he was expecting.
- Chair Reveal: Fujiko uses this to tease Zenigata when he arrives looking for Lupin.
- Clothing Damage: Nami's grandfather cuts away her blouse to reveal the stolen beam she hid on her back.
- Fundoshi: Grandpa wears nothing else, other than a coat when it gets cold.
- Good Old Ways: Nami and Lupin tease Goemon about his. Nami actually calls Goemon an actor from a period drama.
- Hard-Work Montage: Of Grandpa and Nami making one last blade together.
- Pragmatic Adaptation: This episode is a nearly perfect recreation of the original manga chapters, save one part. When Nami goes to visit Goemon, she takes off her clothes and climbs right into bed with him. Goemon leaps out, and Lupin, hearing the commotion, joins Nami in bed instead. When Goemon returns after a cold swim in the ocean, he finds his sword missing. Nami has it, and she's using it fend off Lupin's unwanted advances, via a Groin Attack. It's at this point she's feels the temperature of the sword. The anime tones tones down Lupin's more rape-y overtones.
- Rock–Paper–Scissors: Lupin claims he only wanted to play this with Nami when he pulled her into bed with him.
- What the Hell, Hero?: Lupin interferes because Goemon put his honor over a dying man's dignity.
