Muv-Luv is a Japanese visual novel developed by âge and originally released as an adult game for Windows on February 28, 2003. Consisting of two parts, Muv-Luv Extra and Muv-Luv Unlimited, the gameplay in Muv-Luv follows a linear plot line, which offers pre-determined scenarios and courses of interaction, and focuses on the differing scenarios of the female main characters. Muv-Luv was followed by a sequel, Muv-Luv Alternative, which was released for Windows on April 24, 2006, and follows the storyline of Muv-Luv Unlimited.

The trilogy's story initially presents itself as a light-hearted romantic comedy (Extra), but changes into an alternate timeline coming-of-age story (Unlimited), and finally evolves into an alien invasion war epic (Alternative).

The visual novel Muv-Luv is divided into two parts. The first part, titled Muv-Luv Extra, features playable character Takeru Shirogane (voiced by Sōichirō Hoshi, credited as Tsuyoshi Aiba in the Windows version) who wakes up one morning to find a beautiful young woman sleeping in his bed, just as an infuriated Sumika Kagami (voiced by Hiroko Taguchi, credited as Rika Fujiwara in the Windows version), Takeru's neighbor and childhood friend, comes to wake him up that morning. The young woman, Meiya Mitsurugi (voiced by Kazumi Okushima, credited as Aki Okuda in the Windows version), is the heiress to a financially powerful family and is determined to be Takeru's bride. As the story progresses, both girls compete for Takeru's affections while Takeru has recurring dreams about a marriage promise he made to a girl long ago. At first, he believes the girl was Sumika, but eventually realizes that it was actually Meiya. At the conclusion, Takeru must choose between which of the girls he truly loves. In addition, the player may also make decisions to choose between one of the three girls in the main supporting cast: Chizuru Sakaki (voiced by Masayo Kurata), Miki Tamase (voiced by Hitomi, and Kei Ayamine (voiced by Yūko Nagashima).

The second part, Muv-Luv Unlimited, starts with Takeru Shirogane waking up in his bed, reminiscent of the start of Muv-Luv Extra; however, Takeru quickly realizes that neither Meiya nor Sumika came to wake him that morning. Upon leaving his house, he finds his neighborhood destroyed and Sumika's house crushed under the wreckage of a large mecha. Takeru later learns that he is in an alternate world to Extra, where aliens called BETA (an acronym for "Beings of Extra Terrestrial origin which are Adversary of the human race") have invaded and mankind fights back against the alien aggressors with mecha called Tactical Surface Fighters, (戦術歩行戦闘機, Senjutsuhokōsentōki) usually referred to in Japanese as simply Senjutsuki (戦術機).

Muv-Luv Unlimited explains that the BETA first arrived on Earth in 1973 (in China and Canada) after arriving on the Moon in 1967 and being spotted on Mars in 1958. Subsequent battles with BETA resulted in the global population dropping to 2.4 billion with most of mainland Eurasia having been lost by the time the game starts in 2001. Most of the major characters from the Extra timeline are present in Unlimited, notably excluding the Unlimited world's Takeru, who is said to be dead, and Sumika, who does not seem to exist in the Unlimited timeline. The characters are mostly identical to their Extra counterparts, with the notable exception of Yoroi, Takeru's best friend in Extra who, for reasons unclear, is a girl in this reality.

In the sequel Muv-Luv Alternative, Takeru wakes up three years after the beginning of Unlimited to find himself back in his room. Although he first thinks that the events of Muv-Luv Unlimited were a dream, he soon feels that something is wrong, and leaves the house to find that he has been sent back in time to the beginning of the events in Unlimited. Unwilling to accept the events at the end of Unlimited, he decides to help Professor Kouzuki save the Earth and mankind by bringing the mysterious Alternative IV plan—cancelled at the end of Unlimited—to fruition.

The gameplay in Muv-Luv requires little player interaction as most time is spent reading the text that appears on the lower portion of the screen, representing either dialogue between characters, narration, or the inner thoughts of the player character. Every so often, the player will arrive at a "decision point", and be required to choose between multiple options. Gameplay pauses and depending upon the player's choice, the plot will develop in a specific direction. There are multiple plot lines that the player can follow, one for each heroine in the story, and there are nineteen possible endings. To view all plot lines, the player must replay the game multiple times. The game can end in failure if the player makes the wrong decisions. When this occurs, the player can return to a previously saved spot to try again.

Muv-Luv is divided into two segments, or story arcs—Extra and Unlimited—which serve as different phases in the overall story. Initially, only the Extra arc is available to play, but once the player has completed the Extra scenarios for the two main heroines, Sumika Kagami and Meiya Mitsurugi, the Unlimited arc becomes accessible. The 2016 Steam release removes this lock and makes Unlimited available immediately, albeit with a warning that the player should experience Extra first.