Comic Title: Adventure Comics #305
Publisher: DC (National Periodical Publications, Inc.)
Address: 575 Lexington Avenue, New York, NY
Cover Date: February 1963
On Sale Date: December 27, 1962 Newsstand Date December 27, 1962
Source: House Ad, Superman #159, Page 21

Copyright Date December 27, 1962
Source: Library of Congress, Periodicals 1963

Frequency: Monthly
Cover Price: $0.12
Page Count: 32
Circulation: 415000
Editor: Mort Weisinger

Title: "The Secret of the Mystery Legionnaire"
Pages: 12

Feature(s):
Legion of Super-Heroes (of Earth-1)

Writer: Jerry Siegel
Artist: John Forte

Reprinted In:
Adventure Comics #403 (1971)
Adventure Comics #500 (1983)
Legion of Super-Heroes Archives Vol. 1 HC (1991)
Showcase Presents:Legion of Super-Heroes Vol. 1 TPB (2007)
Legion of Super-Heroes:The Silver Age Omnibus Vol. 1 HC (2017)
Legion of Super-Heroes:The Silver Age Vol. 1 TPB (2018)

Feature Character(s)

• Legion of Super-Heroes (last appearance in Adventure Comics #304; next appearance in Adventure Comics #306)
• Cosmic Boy (last appearance in Adventure Comics #304; next appearance in Adventure Comics #306)
• Saturn Girl (last appearance in Adventure Comics #304; next appearance in Adventure Comics #306)
• Colossal Boy (last appearance in Adventure Comics #304; next appearance in Adventure Comics #306)
• Brainiac 5 (last appearance in Adventure Comics #304; next appearance in Adventure Comics #306)
• Sun Boy (last appearance in Adventure Comics #304; next appearance in Adventure Comics #306)
• Shrinking Violet (last appearance in Adventure Comics #304; next appearance in Adventure Comics #306)
• Bouncing Boy (last appearance in Adventure Comics #304; next appearance in Adventure Comics #306)
• Invisible Kid (last appearance in Adventure Comics #304; next appearance in Adventure Comics #307)
• Mon-El (temporarily uses the aliases "Marvel Lad" and "Legionnaire Lemon" (Lemon is an anagram of Mon-El) in this story; last appearance in Adventure Comics #304; next appearance in Adventure Comics #308)

Villain(s)

• Jax-Ur (last appearance in DC Comics Presents #97; no further appearances)
• Unnamed Phantom Zone criminals (no further appearances)

Other Character(s)

• A Sun-Eater (not to be confused with the Sun-Eater in Adventure Comics #352; no further appearances)
• Antennae Boy (a rejected Legion applicant; next appearance in DC Comics Presents #59)
• Dynamo Boy (a rejected Legion applicant; no further appearances)
• Monsters of the planet Brogg (no further appearances)

Cameo Appearance(s)

• Superboy (in flashback)
• Lightning Lad (as a corpse and as a statue)

Synopsis:
After rejecting two candidates for membership, the Legionnaires meet a worthwhile hero, Marvel Lad, or "Legionnaire Lemon," who demonstrates all the powers of Superboy and Mon-El, but not their weaknesses to Kryptonite and lead. By the end of the day, he passes their initiation by retrieving the rare mineral fluvium, defeating a Sun-Eater, vaniquishing the monsters of the planet Brogg, and creating an anti-gravity metal. After being told that they will induct him the next day, he vanishes.

The following day, he appears – and reveals himself as Mon-El. He explains that Brainiac 5 had secretly developed an antidote to his fatal lead poisoning, but thought it best to keep it a secret in case it failed. Unable to resist the joke, though, the two of them concocted a fictitious identity for him to see whether or not he could join incognito.

After a final trip to the Phantom Zone to bid farewell to its criminal inhabitants, Mon-El joins the Legion permanently.