Comic Title: Justice League of America #20
Publisher: DC (National Periodical Publications, Inc.)
Address: 575 Lexington Avenue, New York, NY
Cover Date: June 1963
On Sale Date: April 25, 1963 Copyright Date April 25, 1963
Source: Library of Congress, Periodicals 1963

Frequency: Monthly, with the exception of Jan., April, July and Oct.
Cover Price: $0.12
Page Count: 32
Editor: Julius Schwartz

Title: "The Mystery of Spaceman X"
Pages: 25

Feature(s):
Justice League of America (of Earth-1)

Writer: Gardner F. Fox
Penciller: Mike Sekowsky
Inker: Bernard Sachs

Reprinted In:
Justice League of America Archives Vol. 3 HC (1994)
Showcase Presents:Justice League of America Vol. 2 TPB (2007)
Justice League of America Omnibus Vol. 1 HC (2014)
Justice League of America:The Silver Age Vol. 3 TPB (2017)

Feature Character(s)

• Justice League of America (last appearance in Justice League of America #19; next appearance in Justice League of America #21)
• Superman (last appearance in Action Comics #301; next appearance in Superman's Girl Friend, Lois Lane #42)
• Batman (last appearance in Detective Comics #316; next appearance in Detective Comics #317)
• Flash (last appearance in Flash #137; next appearance in Flash #138)
• Wonder Woman (last appearance in Wonder Woman #138; next appearance in Wonder Woman #139)
• Green Lantern (last appearance in Green Lantern #21; next appearance in Green Lantern #22)
• Martian Manhunter (last appearance in Detective Comics #316; next appearance in Detective Comics #317)
• Aquaman (last appearance in Aquaman #9; next appearance in Aquaman #10)
• Green Arrow (last appearance in World's Finest Comics #134; next appearance in Justice League of America #21)
• Atom (last appearance in Atom #7; next appearance in Justice League of America #21)

Supporting Character(s)

• Snapper Carr (last appearance in Justice League of America #19; next appearance in Justice League of America #23)

Villain(s)

• An alien metal-being (disguised as a spaceship; no further appearances)
• Spaceman X (a giant robot created by the metal-being; no further appearances)

Other Character(s)

• Humanoid inhabitants of planet Allamar, insectoid inhabitants of planet Ulisson, and plant-people of planet Zomarr (no further appearances)

Synopsis:
Although the Justice League deactivates doom-machines on three planets of the star-sun Mizar, the menaces they caused continue to threaten the inhabitants. Only when the third team of Superman, Batman, and Atom succeeds in their mission do all perils cease, for, unknown to the heroes, each machine had been controlling a menace on a neighboring world.

Meanwhile, disheartened by their seeming failure, the other two teams return to Earth, to confront a giant spaceman, who has inexplicably appeared there. Wonder Woman, Aquaman, and Green Arrow believe the visitor to be friendly, while Flash, Green Lantern, and J’onn J’onzz think that he is a threat, and there is a falling-out. When the third team returns, it is discovered that the giant is really a robot, which has absorbed the energy of the disasters it created and prevented, in order to become invulnerable to the League’s attack. Superman and J’onn J’onzz discover that the robot is controlled by an alien, a living spaceship, which the Man of Steel defeats.