Inspector Zenigata investigates the execution of Arsene Lupin III, but discovers Lupin has used the dead body as a decoy. Lupin flees from the castle via hang glider. In an Egyptian pyramid, Lupin finds Philosopher's Stone with Daisuke Jigen, but they are discovered by Zenigata and the Egyptian police. Lupin escapes the pyramid corridors and Goemon Ishikawa XIII incapacitates the officers surrounding the pyramid. The three use Lupin's Mercedes-Benz SSK to flee from the pyramid. After Lupin delivers the stone to his lover Fujiko Mine, Fujiko turns on Lupin and steals the stone. Lupin listens in on Fujiko to find out who asked for the Stone, only to discover that the Stone was a fake. Dissatisfied that Lupin had given him a fake, the contractor dispatches two of his henchmen to kill the thief. After Lupin and his gang defeat the henchmen, Flinch destroys Lupin's car and his headquarters. Jigen and Goemon berate Lupin for being faithful to Fujiko until Lupin decides to leave Fujiko out. They travel toward the ocean to the desert where Lupin accidentally sets off one of Flinch's booby traps before the group finds a house stocked with food and water. A seriously wounded Fujiko comes for Lupin and alienates Jigen and Goemon in the process. Fujiko inadvertently drugs Lupin to sleep and they are captured by Flinch in his plane. Jigen and Goemon travel to Rome and discover a clue of Lupin's whereabouts, reading "WATER". Jigen gets pursued by Agent Gordon. Surprised at first by Jigen's quick move to threaten him, Gordon manages to bring him and Goemon to Area 64 to be interviewed by Starky, an important government official from America. After listening to a tape of an interfered-with conversation between the secretary general and U.S. president Jimmy Carter, Starky asks for info on the man who broke into the conversation. Jigen hands Gordon the note and angers him by saying that if they knew what the note meant, they wouldn't be where they were. Starky decides that Jigen and Goemon knew nothing, releasing them for the sole purpose of using them to find the mystery man. The two know immediately because of spilled water, and the invisible ink is revealed to be "CARIB". Meanwhile, on a Caribbean island, Lupin manages to find the mystery man, who reveals himself to be a dwarf named Howard Lockewood who refers to himself as Mamo. He offers Lupin eternal life, but Lupin refuses, saying that all he wants is the Philosopher's Stone. He then manages to find it as Jigen and Goemon head for the Caribbean island. Fujiko plays with Lupin awhile, but they are soon chased by thugs led by Frenchy, Lupin having discovered what Mamo's staff was planning to do with the Stone. Lupin and Fujiko end up Mamo's lair, where he reveals that the island is inhabited by clones. After Mamo deems Lupin unworthy of eternal life, Fujiko refuses to abandon Lupin even after Mamo reveals to her his dirty secrets. When Mamo tries to kill him, Lupin is rescued by Jigen, who puts a bullet in Mamo's head, and Goemon. They and Fujiko then get the hell out of Dodge, but not before Goemon engages in a final confrontation against Frenchy. When Goemon learns that even his Zantetsuken couldn't penetrate Frenchy's armor unless it had the intensity of a laser, he instead proceeds to cut Frenchy's face, much to the latter's shock. As Frenchy drops dead into the waters, Zantetsuken is detipped, much to Goemon's sorrow. As Lupin and his gang leave the island, they are chased by Zenigata, who chooses the wrong boat to go after them in. Finding Zenigata on a beach, the Commissioner reveals that Zenigata has been removed from the Lupin case due to circumstances beyond their control. He encourages Zenigata to think of it as an opportunity to take a vacation, but Zenigata instead resigns from the police force and goes after Lupin on his own, deciding that the endgame will be in Colombia. Meanwhile, Lupin and his gang (sans Goemon, who decided to train harder) are thrust into a vision by Mamo after tasting coffee laced with sedatives. Mamo reveals his hand in the history of the world and puts the gang back where he had left them after rearranging the room a bit, as Lupin correctly deduces. When Mamo shows up to reclaim Fujiko, Lupin dares him to prove himself a God by performing some kind of miracle and not one of those Las Vegas-style parlor tricks. Mamo responds by setting off several charges to simulate an earthquake, which registers on the Richter scale and alerts Gordon and Starky to where Mamo is located. Lupin figures out what was behind that earthquake and sets off for the source, where he believes Mamo to be hiding. As Mamo convinces Fujiko to push a button to launch missiles, Lupin shows up as the snake to their Adam and Eve, revealing he rigged the missiles to blow up before they could launch. Frustrated, Mamo takes Fujiko with him to a rocket launching pad and tries fending Lupin off with lasers. Lupin uses the tip of Goemon's Zantetsuken to deflect the lasers onto Mamo and incinerate him, but not before Lupin learns from Mamo that he is the original Lupin and that it was the clone that swung in Transylvania. Lupin is sure that was the real Mamo until he discovers a chip hidden among the ashes. The rocket emerges, containing a brain that reveals itself to be the original Mamo. Lupin figures out that Mamo had controlled the clones resembling his old body right before the rocket launches into space as Mamo declares he is setting off into space and eventually plans to return to Earth as its one true God. Lupin and Fujiko escape the rocket's trajectory, but not before Lupin puts an explosive onto the rocket. The glass shatters, and Mamo's gigantic brain drifts toward the sun as Lupin reminds Mamo that time does everyone in and that he should be grateful Lupin put him to death when he did. Fujiko reveals that Zenigata had threatened her into luring his quarry for the catch, and Lupin is unable to convince Zenigata that he is a clone and that the real Lupin was the one that was hanged. Fujiko offers to help Lupin after they kiss, but Gordon launches his missiles on Mamo's Colombian base, and Jigen picks up Fujiko in a plane while Zenigata and Lupin escape together on foot.
The Japanese release was handled by Toho, who was also responsible for the initial English dub, also known as the JAL dub or the HK dub produced in 1978, and was shown on JAL Airline flights. The English dub was also released to several markets in Europe, and was also screened at least once theatrically in the United States. The second English dub, handled by Streamline Pictures, was released to home video by Orion Home Video in late April 1995. It was later released on DVD by Image Entertainment in 1998, but without the Japanese language track and under the name The Mystery of Mamo. The third English dub, which has never been released in the US, was created and released on home video in the UK by Manga Entertainment, Ltd. in 1996 and renamed Secret of Mamo. The fourth dub was created by Fuse Entertainment and released to DVD by Pioneer Entertainment (now Geneon Universal Entertainment) in 2003 as The Secret of Mamo. This release was duplicated for Madman Entertainment's DVD release for Australia and New Zealand, and for Manga Entertainment's DVD release for the UK. In 2012, TMS Entertainment began streaming the Geneon dub of the movie on Hulu. In 2013, Discotek Media released The Mystery of Mamo on DVD in the US on February 26, 2013 with all four English dubs, including a reconstruction of the 1978 English dub.
- Arsène Lupin III: Yasuo Yamada
- Howard Lockewood (Mamo): Kō Nishimura
- Fujiko Mine: Eiko Masuyama
- Daisuke Jigen: Kiyoshi Kobayashi
- Goemon Ishikawa XIII: Makio Inoue
- Inspector Koichi Zenigata: Gorō Naya
- Heinrich "Starky" Gissinger: Tōru Ōhira
- Special Agent Gordon: Hidekatsu Shibata
- Police commissioner: Kōsei Tomita
- Flinch: Shōzō Iizuka
- Scientist: Ichirō Murakoshi
- Premier: Shunsuke Shima
- Officer: Yūji Mikimoto
- Egyptian police chief: Haruo Minami
- President Jimmy Carter: Fujio Akatsuka
- Boris (Chief secretary): Ikki Kajiwara
Frontier Enterprises dub
NOTE: The below cast is incomplete due to Frontier Enterprises not documenting its English dubbing work properly; those cast members marked with an asterisk are to be treated as speculation based on voice recognition until further notice.
- Foward Fughes (Mamaux): Mike Worman*
- Margo: Patricia Kobayashi*
- Samurai: William Ross
- Detective Ed Scott: Gregory Starr
- Special Agent Gordon: Don Knode*
- Police commissioner: Don Knode*
Streamline Pictures dub
- Lupin III: Bob Bergen
- Hayward Lockewood (Mamo): Robert Axelrod
- Fujiko Mine: Edie Mirman
- Daisuke Jigen: Steve Bulen
- Goemon Ishikawa XIII: Kirk Thornton
- Inspector Koichi Zenigata: David Povall
- Heinrich "Starky" Gissinger: Steve Kramer
- Special Agent Gordon: Michael Forest
- Police commissioner: Jeff Winkless
- Egyptian police chief: Steve Kramer
- President Bill Clinton: Steve Kramer
- Boris (Chief secretary): Steve Kramer
Manga Entertainment, Ltd. dub
- Wolf the Third: William Dufris
- Howard Lockewood (Mamo): Allan Wenger
- Fujiko Mine: Toni Barry
- Daisuke Jigen: Eric Meyers
- Goemon Ishikawa XIII: Garrick Hagon
- Inspector Koichi Zenigata: Sean Barrett
- Heinrich "Starky" Gissinger: John Baddeley
- Special Agent Gordon: William Roberts
- Police commissioner: John Baddeley
- Egyptian police chief: John Baddeley
- President: Sean Barrett
- Boris (Chief secretary): William Roberts
Geneon Entertainment dub
- Arsène Lupin III: Tony Oliver
- Howard Lockewood (Mamo): Paul St. Peter
- Fujiko Mine: Michelle Ruff
- Daisuke Jigen: Richard Epcar
- Goemon Ishikawa XIII: Lex Lang
- Inspector Koichi Zenigata: Dan Martin
- Heinrich "Starky" Gissinger: Osgood W. Glick
- Special Agent Gordon: Michael McConnohie
- Police commissioner: Richard Cansino
- Flinch: Bob Pappenbrook
- Scientist: Richard Cansino
- Premier: Richard Cansino
- Egyptian police chief: Richard Cansino
- President George W. Bush: Richard Cansino
- Boris (Chief secretary): Richard Cansino
- In Japan, the film was promoted under the title Lupin vs the Clone. While this title differentiates it from other "Lupin III" films, it gave away an important plot point, and most fans were thus uncomfortable with using it freely. In (partial) consequence, upon release in America the film was given the spoiler-free title "The Mystery of Mamo."
- Mamo's voice actor, Ko Nishimura performed his role in a deep masculine voice to contrast with Mamo's miniature appearance. This was a deliberate decision made to astonish the audience, who would hear only Mamo's voice in the first part of the film and would only later see him appear in person. This effect was replicated by voice actor Robert Axelrod in the American release.
- Production on the film took fifteen months.
- A total of 62,000 cell sheets were used.
- There were 575 pages of storyboard.
- There were 196 character design sheets for this movie.
- There were 1,315 people in the staff.
- Jigen fires his gun only three times in this film.
- This is Monkey Punch's favorite Lupin movie.
- Holds the record for being the only Lupin III anime to be dubbed more times than any other Lupin related anime.
- This was the first Lupin III anime to ever be dubbed into English.
- Originally the opening of the movie was to have Zenigata believing that Lupin died in a shipwreck, but was ultimately change to him believing that Lupin got hang at the gallows.
- This was the last time where Zenigata's full name was Inspector Heiji Zenigata VII. While in his later anime appearances it was established that his full name was Inspector Koichi Zenigata rather than sharing the same name as his ancestor.
- The "Tele-Pach" orange rock candies that Lupin was seen eating are an actual product similar to Pop Rocks. Yasuo Yamada was the announcer for the Tele-Pach TV commercials during the 70's.
- 18,000 reference photos were used for background and mechanical items.
- 218 colors were used in this movie.