Mario was once a great hero until he decides to go into retirement after Luigi is murdered. Now living under the name Michael Camorelli, he spends his days doing little until the crime in Ferral City rises and he is told that the Koopa killed Luigi. Realizing he must do something, Mario returns back into the spotlight after over 20 years in retirement, hunting the street gang known as the Goombas and their mysterious leader Koopa.
- Rich Alvarez as Mario / Micheal Camorelli / Radio Host in Dream Sequence / The Darkness / Goomba #1 / Koopa
- Matt Provencal as Bill Summers
- Chris Muller as Lieutenant John Gibbs /Goomba #2 / Luigi / Luigi Jr. / Blue Hood
- Doug Orofino as Dr. John Horus
- Tim Muller as Ness
- Jackie Thompson as Denise Wright / Goomba #3
- Scott Otter as Mayor Phillip Carson
- Christina Folsom as Ferrah Wilson
- Ed Gyles, Jr. as Jeckle
- Garrett Allen as Clause / Wrathnar
- Katherine Folsom as Female Bank Manager
- Kris Loewen, Nick Lunetta, and Tabatha Smart as Citizens of Ferral City
- Joel Edelman
- Claudia Stelmach as Undercover Agent Working for Gibbs / Goomba #4
- Kevin Chamberlain as Goomba #5
- Josh de Guzman as Goomba #6
- Tina Bird as Princess Rosalina
- The series is inspired largely by several Batman stories, most prominently being Frank Miller's The Dark Knight Returns.
- When asked on whether or not the series is a sequel to Stupid Mario Brothers, Rich Alvarez simply replied, "Maybe." When the Darkness appeared and Mario and him (in disguise of the Koopa) got in a lightsaber fight, fans were convinced that The Plumber Knight Returns is indeed a sequel. This was later confirmed by Alvarez himself that it is a full-fledged sequel.
- Doug Orofino's character only appears in 3 episodes and gets killed off in the last one featuring him. Alvarez said that this was done because of the fact that Orofino was only available for a short period of time. However as of season 2, Orofino has returned for 3 more episodes, and his character is revealed to have survived.
- The Goombas are not in any way linked to the mushroom enemies from the game, Alvarez just wanted to "pay an homage to the games and the classic enemies". They also have an attitude like the Jokerz from Batman Beyond, though the Joker (or a similar character) had not been introduced to the series so far. Some fans wanted to see a Joker-esque character in the series to conflict with the serious style (like the Joker did in the Batman franchise) and having a sense of humor to lighten up the dark and grim setting.
- A fan theory was that Wario was somehow going to be the Joker-esque character due to his comical nature. However, this theory was proven wrong when it was revealed that Mario had managed to change Wario to his side, referring to him as "the greatest friend he ever had."
- In episode 10, the Koopa revealed himself to be the Darkness. This was due to after he fused his spirit with Mario alongside Link and Merlin for his battle with Gannondorf, the 3 spirits left Mario's body. With him returning to his base form, Link being resurrected to Linkle, and Merlin passing into the afterlife.
- When the Koopa reveals himself to be the Darkness, he starts laughing much like the Joker does. This was to have the Darkness more reinvented to be the "Joker" for this series.
- The character of Luigi Jr. was only created to fill in the role of Bill (Wario and Mona's son) due to Matthew Thomas Provencal moving away and unable to make it to production of the second season.
- This series has the most delayed episodes of the entire Stupid Mario related series
- Season One's finale was delayed by an extra week, while the second half of the second season, the episodes were released by bi-weekly due to the amount of special effects required
- The Season Two and also Series Finale is currently the longest delayed episode by over 4 months beating out Bloody Confrontations and The Super Mario Brothers which both were delayed by over 3 weeks each.
The Plumber Knight Returns season 1 received majority positive reviews with fans praising the story and character development. While fans were split to the middle over the Koopa being the Darkness resurrected yet again, with either being happy he was back and some on how overused the character was.
The second and final season received mixed reviews though praise was given towards Julian Petruzzelli as Commissioner Steve McGuiness, along with the character's development and role in the story. The majority states that too much was crammed into the season with Jekyll's rise to power, the Blue Hood's arrival, and the Darkness's return all being shorted to fit into one season. Some considered the inclusion of Luigi Jr. to be odd and out of place considering he appeared out of nowhere (This was due to Bill being cut from the story).
- Teaser Trailer
- Production Diary 1
- Official Trailer
- Production Diary 2
- Production Diary 3
- Episode 10 Trailer
- Season One Bloopers
- Season 2 Teaser