In a skirmish between the Regular Army and Rebel Army, the Time Slug General Morden was operating gets heavily damaged and sends everyone in a parallel dimension. Now the Regular Army has to join their dimension's wide cast of enemies to get back home.
Gameplay was similar to SNK's previous two games (Metal Slug Defense and Metal Slug Attack) where a team of units attacked their enemy base while protecting their own. However, most of the battle was done in the background, and the player only managed the various currencies to level up their units.
The game servers shut down on September 30 2021 and the game is no longer playable
Metal Slug Infinity provided examples of:
- Allegedly Free Game: Gems were the game's premium currency and are used to promote units, Slugs, War Books, and Artifacts to stronger levels. Unlike Metal Slug Defense and Metal Slug Attack, Metal Slug Infinity was more generous in giving Gems of decent quantities per day. Likewise, the player was easily able to earn Units and Slugs through the various rewards, events, and submodes.
- Announcer Chatter: Sophia Greenville, voiced by Mina Tominaga note , talked in the background. Her voice lines were entirely in Japanese, but there were no subtitles.
- Art-Shifted Sequel: Instead of using pixel sprites, Infinity used a stylized chibi look that fully resembled the arcade graphics. This was because the units could aim at various angles and thus using pixels would make the animations harder to draw.
- Attack Animal: The animal Slugs from the main games all made an appearance in this game. The mutated enemy creatures were also playable.
- Backtracking: One of the main gameplay element was called Returning. Should the player be overwhelmed, they could return a couple of thousand stages back. While their gold, soldier levels, and Gold Mines were reset, the player earned medals to boost the unit's elemental type mastery, gold rates were increased, and was rewarded with various prizes.
- Car Fu: Once a Slug ran out of fuel or health, the vehicle started to flash red. The player could tap the Slug to make it ram onto the closest enemy unit and explode. Animal Slugs simply slammed the closest enemies and ran back to base.
- Endless Game: The main game continued forever without an ending in sight until it was shut down.
- Enemy Mine: The player could deploy Rebel Army, Amadeus Syndicate, Ptolemaic Army, and other enemy independent units and vehicles.
- Evil Doppelgänger: On reaching stage 10,000, the heroic Regular Army troops joined their enemies in attacking the player.
- Excuse Plot: The game began with everyone accidentally traveling to another dimension. Nothing happened after that, and it was just them fighting their way out.
- Fast-Forward Mechanic: Units with the Stimpack ability increased the chance to speed up the battle. Others had the ability to skip a number of stages. This was useful when Returning as the units would quickly travel to the maximum stage they were at and could skip boss stages at the right moment.
- Instant Armor: Some units eventually produced a temporary barrier that granted immunity against a specific elemental type. Enemy units had a permanent barrier in their disposal.
- Multi-Mook Melee: The Battle of Rift stages had various enemy units bombard from both sides of the arena. As the battle progressed, the enemy units and their projectiles become bigger in size.
- No-Sell: The three factions had an immunity to a certain effect.
- Regular Army units were immune to knockback.
- Rebel Army units were immune to shock.
- Another Force units were immune to stun.
- One Size Fits All: Everyone could ride any Slug no matter their size. Large units either shrank in size or were crammed inside the cockpit.
- Original Generation: Like Attack, Infinity had new units and Slugs created for this game and were marked with the "MSI Origin" label in their description. The exception was the Turtle Slug who was an unused Metal Slug 3 concept.
- Promoted to Playable: Cynthia Hartnett, a combat instructor in Metal Slug 7/XX, fought in this game using a Big Rocket Launcher.
- Speedy Snail: The Big Snail was quite fast and would spin to get closer to the enemy.
- Tactical RockPaperScissors: Regular Army > Rebel Army > Another Force > Regular Army.
- Timed Mission: Most submodes had the units clear the main target under a time limit. The main game had certain boss stages that has to be cleared in under 300 seconds.
- Villain Team-Up: The Rebel Army also consisted of Amadeus Syndicate and Ptolemaic Army units.
- Warp Whistle: The Slugs had the ability to warp a number of stages ahead. This is useful when the player was returned a thousand stages back. The Slugs could not warp in boss stages nor any stage past the previous stage breakthrough.