Battletoads/Double Dragon is, as the name suggests, a crossover between Battletoads and Double Dragon. The game is developed by Rare and published by Tradewest for the NES.

The city-sized battle-cruiser called the Colossus plows out of the moon with an actively glowing laser cannon with Professor T. Bird finding it approaching Earth on his radar. He realized that the Colossus, occupied by the combined forces of the Battletoads' foe the Dark Queen and the boss of the Shadow Warriors, known as the Shadow Boss, plan to invade Earth and wreak mayhem across it, now it is up for the Battletoads to team up with the Lee Brothers (a.k.a. the Double Dragon) in order to fight back the dark forces and save Earth using their Smash Hits and martial arts to take down all foes who stand in their path of saving the world.

As a deviation from prior Battletoads games, you can now pick from all three toads, all whom are not kidnapped this time, as well as Billy and Jimmy Lee as they go through seven stages which include travelling on the tail of the Colossus, infiltrating its interior, fighting in space ala Asteroids, stopping a planet-busting missile, infiltrating a gangster hideout, and reaching the Dark Queen's escape vessel in the combined team's efforts to save the day from both the Dark Queen and the Shadow Boss. There are three types of gameplay that include three-dimensional gameplay and traditional platforming. The game also is notable for a variety of continuity issues with Double Dragon despite getting some like Abobo right. Such errors include Willy Mackey being misnamed Roper and a variety of Shadow Warriors receiving Remember the New Guy? treatment such as the Shadow Boss himself, yet he does vaguely resemble Burnov.

Tropes:

• Auto-Scrolling Level: The famous Battletoads Turbo Tunnel stage returns in Stage 2, with the heroes riding hoverbikes that automatically go forward through an obstacle course, though it's not as fast or difficult as in other games.
• Batman Can Breathe in Space: All the characters are shown to be able to walk around on the outside of spaceships with no issue, which while not that unusual for a Battletoads game, is a bit out there compared to other Double Dragon games.
• Battleship Raid: Roughly two thirds of the game consist of the heroes raiding the Dark Queen's Colossus Ratship. They must get onboard, infiltrate it, steal an escape pod to destroy it, and hijack the giant missile it shoots.
• Big Bad Duumvirate: The Dark Queen teams up with the Shadow Boss for an assault on Earth, forcing both of their enemies to also team up to stop them.
• Breaking Old Trends: None of the Battletoads get kidnapped this time and are all playable from the start as opposed to one or two of them being kidnapped depending on the game, though due to every version of the game only supporting up to two players, all three can't be controlled at the same time, unlike the later-released arcade game.
• Call-Back: Just as in most versions of the first Double Dragon game, Abobo is the first boss to be faced by the heroes.
• Canon Discontinuity: It's not listed as a canon Double Dragon game. Subsequent games in that franchise ignore the events of this game.
• Chuck Cunningham Syndrome: The game never elaborates on why some Double Dragon characters such as Jeff or Chin Taimei did not appear.
• Decomposite Character: As Jimmy remains a hero with zero indication of his evil from the NES version of the first game or the cartoon, the crossover instead attributes the "Shadow Boss" title to an original character it presented as the Lee Brothers' archfoe.
• Friendly Fireproof: Playing a 2-player game in mode B will prevent the players from being able to damage each other.
• Guest Fighter: Although Battletoads & Double Dragon is a crossover, the game engine is pure Battletoads game mechanics.
• No Fair Cheating: If you use the level select cheat code, the ending changes to Dark Queen (or T. Bird in the SNES version) accusing you of cheating and insisting you play the game from start to finish.
• Palette Swap: In essence, there are only two movesets: A Battletoad and one half of the Double Dragons. Rash, Zitz and Pimple have the same gameplay animation, while Billy and Jimmy share the same moveset. The two pairs have the same Oh, Crap! animation (The Battletoads have Rash's Jaw Drop^note Which is quite jarring, since the game is also available in SNES, but Battletoads in Battlemaniacs also introduces the trend of 'unique Oh, Crap! animation', with Pimple's animation being 'biting his fingers', while the Double Dragons have 'point finger and have an Eye Take'.^note The Eye Take is later given to Zitz in the arcade game). What differentiates each character is just the color of their sprites.
• Pistol-Whipping: Roper will smack any characters who get too close with his machine gun, and it hurts a lot.
• Remember the New Guy?: Shadow Boss, one of the Big Bad Duumvirate. He's presented as the Lees' archenemy, despite the fact that he's never appeared in a video game before (he's based on the Shadow Master from the Double Dragon cartoon, which only superficially resembles the previous video games).
• Retcon: Machine Gunner Willy is inexplicably called Roper, which is a mix-up with an unrelated enemy.
• Storming the Castle: After the heroes finally arrive on Earth, Stage 6 involves tracking down Shadow Boss in the Shadow Warriors' hideout.
• Taking You with Me: Strongly implied after the Colossus is blown to smithereens. The Dark Queen and Shadow Boss get on a missile that is set to destroy the Earth, so the players must force it to make a crash landing.
• Unexpected Gameplay Change: Stage 4 has the players pilot ships to shoot down the Dark Queen's fleet.
• Variable Mix: Pausing in the NES version causes the melody lines of the music to drop out, leaving just the drums and bass like Super Mario Bros. 2 before it.