The gameplay consists of the player reading the game's story, at certain points making choices that affect its direction, leading to one of several possible endings.

The development team included director Takumi Nakazawa, writer and planner Kotaro Uchikoshi, character designer Yuu Takigawa, and composer Takeshi Abo.

A remaster that uses the remake as the basis, but brings back the 2D visuals, is set to release worldwide for Nintendo Switch, PlayStation 4, and Windows in March 2025.

[3] Ever 17 is set in Japan, in the underwater marine theme park LeMU, 51 meters below the surface of the artificial island Insel Null.

After an incident, almost half of LeMU is flooded, and the path to the surface and the communication lines are cut off, trapping the game's characters inside.

The player takes the roles of two characters, and sees the story from their respective perspectives: Takeshi Kuranari, a student who visited the park with his friends but got separated from them; and an amnesiac boy who does not remember his own name, and is simply called "the Kid".

The game opens on May 1, 2017, when a blackout occurs in LeMU, its communications system go down, and the park springs a leak; portions of it get flooded, blocking the exit and trapping the characters.

If the player plays as Takeshi, he builds a relationship with Tsugumi, who reveals that she and her pet hamster Chami are carriers of the Cure virus, which has rewritten their genetic code and halted their biological aging.

It is revealed that LeMU was created by the Leiblich Pharmaceutical company as a cover for IBF, a research facility underneath the park.

They find a submarine and escape, but its batteries die; Takeshi exits it, giving it enough buoyancy to save Tsugumi, while he drowns on the ocean floor.

The group manages to contact the surface, and open and close doors to rooms in LeMU to move the water around, allowing them to escape.

[k] The Kid of 2017, whose name is Ryogo Kaburaki,[l] played the role of Takeshi during the recreation, having stopped aging due to the Cure virus.

Blick Winkel travels to 2017, and wakes up Takeshi on the seafloor, forcing him to swim to IBF; inside, he injects Coco with Tsugumi's antibodies.

Sora, who now has a robotic body, is given back her memories from the disk, and Yubiseiharukana leaks information about Leiblich, exposing them as the ones behind the Tief Blau outbreak.

The game was developed by KID,[2] with direction by Takumi Nakazawa,[4] planning and scenario writing by Kotaro Uchikoshi,[2] character design by Yuu Takigawa,[5] and music by Takeshi Abo.

[7] Uchikoshi, who was freelancing at the time, came up with the original concept and pitch for the game after having started to research material in August 2001, including the Russian mathematician P. D. Ouspensky's book Tertium Organum.

The writing process for all routes were supervised by Uchikoshi, in an effort to eliminate inconsistencies in the setting or characters, and plot holes between the different parts of the story.

[7] The writers only put little effort into hiding the 17-year gap between the Kid's and Takeshi's routes, writing each day of the story casually without a particular plan in mind.

Rather, it took special effort to make it seem convincing when the game's secrets were revealed, with hints toward the truth added throughout the story, such as Tsugumi acting more suspicious when playing as the Kid due to having already experienced the events of Takeshi's route, or slight differences between the Kid's and Takeshi's routes due to the characters not being able to perfectly recreate events.

While Coco was planned from the beginning as the character who knew the truth, Uchikoshi did not come up with the idea of leaving her on the bottom of the ocean until during the writing of the script.

When Uchikoshi considered how to bring characters over from 2017 to 2034, some worked out naturally: Tsugumi was immortal due to being infected with the Curé Virus, You had a clone in 2034, and Sora was an artificial intelligence.

[10] When a PlayStation Portable version was developed, Nakazawa was asked to create new content for it; as it would have been hard to expand the story or add new event computer graphics artwork (CGs), it was decided to create an in-game dictionary explaining terms used to allow players to enjoy the game on a deeper level.

Additionally, Uchikoshi had visited Germany and was influenced by the 1999 animated film Jin-Roh: The Wolf Brigade, and how the friend he had the theme park conversation with spoke German.

The phrase "The protagonist of bishoujo games is the player themselves" stuck with him due to the impact the criticism had on him, leading to Blick Winkel's creation.

[10] Sora was planned as an artificial intelligence from the start, but because Uchikoshi thought robots seemed cliché, and holograms could not move around to different places, he came up with the idea of her being an image shot onto people's eyes with a laser; thinking that if a layman like himself could come up with the idea of such technology, an expert in the field could have been working on developing it already, so he researched it and found out about retinal scanning display technology, and made use of it in the game's story.

Sara was noted by Uchikoshi as being the otherwise single normal person in the cast, offsetting the balance; because of this, she was made into the Kid's twin sister.

[7] The event CGs were decided by Nakazawa based on the game's plot and script, and written down in a specification document given to the art team.

Prior to composing the music, he read through the game's story, to understand the setting and each character's personality as much as possible.

[22] A manga adaptation of the game was drawn by Chigusa Umetani and released by Enterbrain on their Famitsu Comic Clear website from 2011 to 2012; it has since been collected in two volumes.

[3] Jason Young at GameZone called the game an "all-age masterpiece [that] is nearly flawless in every regard", and said that it was the best visual novel that was available in English.