Code Name: S.T.E.A.M.

M. is a turn-based strategy game controlled in the style of a third-person shooter, similar to Sega's Valkyria Chronicles series. M." facing off against an opposing team of alien invaders.

For the player, both movement and attacking requires the use of "steam", a resource that depletes whenever a character moves around or uses their weapon, in the latter case depending on the type of weapon be used. By saving up the same amount of steam required for the character's weapon to be used, certain characters can perform "overwatch attacks" during the opponent's turn, letting them attack enemies that wander into their line of sight with the added potential to stun them for the rest of the turn. However, the opposing alien team can also perform this strategy, so caution is encouraged. Before each level, the player selects up to four characters, with more becoming available as the story progresses.

The game's narrative framing device is that it is a comic book being read to the player. The story opens in a steampunk fantasy version of London on the opening day of the Steamgate Bridge. Henry Fleming (based on the protagonist of the same name from the novel The Red Badge of Courage and voiced by Adam Baldwin) is going over security detail at the American Embassy, when suddenly the city is attacked by an unknown enemy force. He manages to escape and meet up with his old friend John Henry (voiced by Michael Dorn).

The two are then rescued by an airship called the Lady Liberty, captained by the once-presumed-dead President Abraham Lincoln (voiced by Wil Wheaton). M. (short for Strike Team Eliminating the Alien Menace).

This game marks the directorial debut of Paul Patrashcu, who had been with Intelligent Systems for eight years at that point. As with all of Nintendo's outside collaborations, the project was overseen by Nintendo's Software Planning & Development division under veteran producer Hitoshi Yamagami, who also oversees development of series such as Fire Emblem, Pokémon, and Xenoblade Chronicles. The first pitch began with the words "Steampunk Civil War", according to the director's explanation at Nintendo's E3 2014 developer roundtable where the game was first unveiled to the press.

Yamagami and Patrashcu, the latter of whom had a personal fondness for strategy games, wanted to make a game that was accessible to a broader group of people who previously found strategy games unappealing by removing certain abstractions that had been established in the genre, such as a heavy reliance on overhead maps and a general distance from the character units themselves. Thus the idea was born to use an over-the-shoulder third-person camera that would serve as the player's only way of seeing around the map, directly putting them in the respective character's shoes. That is also where the influence from third-person shooter games comes into play, with the direct control over the aiming reticle of the character's guns being in the player's hands, just like in a traditional shooter game. All of these mechanics were blended together under the familiar turn-based system of other strategy games, like Intelligent Systems's own Fire Emblem and Advance Wars series, or contemporaries such as XCOM and Valkyria Chronicles, resulting in a strategy game which aims to offer both a lot of depth and be welcoming to previously disinterested players.

M. has earned aggregate critic scores of 68% from GameRankings and a 69 out of 100 from Metacritic, indicating "mixed or average" reception. Ben Moore from GameTrailers called it "a complex game that's hard to put down," praising the varied gameplay and character progression systems, and the game's "absurd moments that keep things entertaining". GameRevolution's Ryan Bates enjoyed the level strategy that fans of the genre will "eat up" while "non-strategy gamers will find it easier to understand and get into than other current offerings".

Henry Gilbert from GamesRadar praised the design and setting of a "unique world full of memorable characters," the gameplay mechanics, and "the odd team of steampunk weirdos really mixes up the gameplay to make for some impactful action, and the dense maps belie a raft of colorful design". Kimberley Wallace from Game Informer said the game's "biggest assets are its variety and unpredictability" in missions and strategies. IGN's Jose Otero enjoyed the design of the characters and gameplay options, calling the arsenal itself "wacky, but also extremely deep," though he felt that the enemies were less memorable visually than the playable characters. Otero noted the lack of an overhead map or view feature, feeling that it made players "have to think carefully" to avoid ambushes and make counters.

Code Name: S.T.E.A.M. was the first video game to debut Marth's current English voice actor, Yuri Lowenthal, who has since reprised his role in all future English-speaking appearances.

The game also received minor representation in the 2018 Nintendo Switch crossover fighting game Super Smash Bros. Ultimate. Henry Fleming appears as a Spirit and four music tracks from the game were included, including three tracks based on themes from the Fire Emblem series.