Crash Twinsanity is a 2004 platform game developed by Traveller's Tales and published by Vivendi Universal Games for the PlayStation 2 and Xbox. It is the eleventh installment in the Crash Bandicoot series and the fifth game in the main series. The game's story takes place three years after the events of Crash Bandicoot: The Wrath of Cortex and follows the main protagonist and antagonist of the series, Crash Bandicoot and Doctor Neo Cortex, who must work together to stop the Evil Twins — a pair of interdimensional parrots — from destroying N. Sanity Island.

Crash Twinsanity began development as Crash Bandicoot Evolution, and was intended to have a more serious tone than previous games. The similarity of the game's premise to Ratchet & Clank convinced Traveller's Tales to restart production and create Crash Twinsanity as a comedic title in which Cortex would be teamed up with Crash as a means of exploring his character. The game marks the debut of Lex Lang as the voice of Cortex, replacing previous actor Clancy Brown. Various concepts were removed from the game during production due to time constraints, but were incorporated into the final game as unlockable extras.

Crash Twinsanity received mixed reviews upon release; while it was mostly praised for the gameplay variety, presentation, narrative, humour and Lang's voice performance, there were mixed reactions towards the music and criticism for the controls, platforming sections, and camera. Two mobile versions of the game were subsequently released and were also met with mixed reception.

Crash Twinsanity differs from previous entries in the series through its free-roaming gameplay style; the player can explore the game's environments in all directions and travel seamlessly between levels. As opposed to the "hub room" format utilized by previous games, the player's progression through Twinsanity is influenced by the plot. Over the course of the game, the player controls three different characters: Crash Bandicoot, Doctor Neo Cortex (or both at the same time) and new character Nina Cortex; the controlled character alternates between particular levels. Crash is the primary player character and has the ability to jump into the air and land on enemy characters, spin in a tornado-like fashion to knock enemies off-screen, slide across the ground and perform a body slam to break certain objects. Cortex is armed with a raygun and a limited amount of ammo, and Nina uses her mechanical arms to punch enemies and latch onto distant ledges. When Crash and Cortex are together, Crash can use Cortex to increase the range of his spin attack, use Cortex's head as a hammer to break certain objects, and hurl Cortex across gaps to activate switches. Two levels in the game incorporate a gameplay style named "HumiliSkate", which involves Crash riding Cortex as a makeshift snowboard, while two others are played in a style named "Doc Amok", which involves Crash clearing a path for a fleeing Cortex. Another gameplay style, named "RollerBrawl", involves the player steering Crash and Cortex through the level as they engage in a brawl.

Wumpa Fruit is scattered throughout the game, and grant an extra life if 100 Fruits are collected. Crates also carry Wumpa Fruit, which can be obtained if the crates are broken. TNT crates explode after a three-second fuse when jumped on, while Nitro crates explode upon any physical contact. Crates with plungers will detonate any Nitro crates in the immediate vicinity. Checkpoint crates allow the player to return to a specific point in the stage upon losing a life, while World crates save the player's progress. Gems littered throughout the levels can be collected to unlock extra content such as concept art and storyboards.

Three years after his previous defeat by Crash, Doctor Neo Cortex returns to the Wumpa Islands to exact revenge on Crash. Cortex incapacitates Crash's sister, Coco, and impersonates her to lure Crash into a trap. After Crash's victory against Cortex and his Mecha-Bandicoot, Cortex and Crash are both sent plummeting down a hole and land in a cave. Enraged by his defeat, Cortex attacks Crash, and the pair engage in a prolonged fight across the cave. Upon returning to the surface, Crash and Cortex encounter a pair of interdimensional parrots named the Evil Twins, who plan to destroy the Wumpa Islands and steal Cortex's brain. After Cortex pleads for Crash's help, he is spontaneously attacked by bees and finds himself captured by Papu Papu and his subordinate tribesmen after stumbling into their territory. Crash rescues Cortex from captivity and escapes a pursuing mob of tribesmen. Crash and Cortex have another encounter with the Evil Twins, who bring a deity statue to life to attack the pair. Cortex, having learned that the Evil Twins come from the Tenth Dimension, concocts a plan and summons Crash to his antarctic lair.

Crash and Cortex attempt to enter the Iceberg Lair through the front entrance, but the door is frozen shut, forcing the pair to take an alternate path inside. They inadvertently free Uka Uka in the process, who attacks them with a giant body formed from ice. Uka Uka is convinced by his twin brother Aku Aku to join forces and attack the Evil Twins themselves, but both are easily defeated. Cortex introduces the Psychetron, a device that will allow travel to the Tenth Dimension, but requires Power Crystals to function. Crash uses Cortex as an impromptu snowboard in an attempt to reach Doctor N. Gin's battleship and gather the Power Crystals, and destroys Dingodile's shack in the process. Crash's venture through the battleship eventually results in an explosion of a cache of TNT crates, which sinks the ship and propels Crash into a confrontation with Doctors Nefarious Tropy and Nitrus Brio on a distant ice floe. Crash returns to the Iceberg Lair with Cortex, where the latter is attacked by a recovered Coco, who believes that Cortex kidnapped Crash. Coco's assault sends two of the Power Crystals flying into the Psychetron, which damages the machine and paralyzes Coco in a chain reaction. Crash and Cortex set a course for Madame Amberly's Academy of Evil in the hopes of recruiting Cortex's niece Nina to assist in repairing the Psychetron.

Crash and Cortex sneak into the Academy through the sewer system, where they fend off an attack from Dingodile. After Cortex finds Nina and has an encounter with Madame Amberly, he recalls the origin of the Evil Twins; when Cortex was an 8-year-old student in the Academy, he used his two pet parrots Victor and Moritz as test subjects for a prototype of his Evolvo-Ray, but the experiment resulted in the parrots being transported to the Tenth Dimension, where they would be mutated by the environment's severe "reverso-radioactivity". The trio return to the Iceberg Lair, repair the Psychetron and travel to the Tenth Dimension to face the Evil Twins. Upon their arrival, Nina is kidnapped by an evil doppelganger of Crash and taken to his desolate home on Twinsanity Island. After cornering Evil Crash, Cortex valiantly offers himself in Nina's stead, which leads to a chase. Crash, Cortex and Nina escape Evil Crash and make their way to the Evil Twins' compound, where Cortex confronts the Evil Twins and commands them back into their cage. The Evil Twins transform their cage into a giant robot and engage in a final battle with the trio. The Evil Twins are defeated and flee the compound, only to be devoured by Evil Crash when they take refuge in his home. After the trio return to their own dimension, Cortex attempts to eliminate Crash, but the malfunctioning Psychetron teleports Cortex into Crash's mind, where he is trapped with a crowd of dancing Crash duplicates.