Setting

Signalis takes place during an era with space travel and advanced technology, in an unidentified planetary system that is governed by the totalitarian Nation of Eusan, which remains at war with a larger Empire from which it broke away. The political and economic center of the Nation of Eusan is the Heimat colony, but it maintains outposts on other nearby worlds. Eusan employs Replikas, androids made with copied human neural imprints, as the bulk of its military and labor force. A consequence of using neural imprints is that Replikas inherit various idiosyncrasies from them, such as a compulsive desire to listen to music or bathe, and failure to indulge these idiosyncrasies can cause a Replika to destabilize and cease functioning. Natural-born human commanders called Gestalts manage and give direction to Replikas.

Plot

A scout shuttle known as the Penrose-512 crashes on an unknown planet. An LSTR ("Elster") type Replika unit awakens and searches for her missing Gestalt partner, Ariane. She leaves the ship and crosses a rectangular arch, finding herself in front of a hole with a staircase leading down. After she descends and crawls into a tight tunnel leading into a room, the hole closes behind her. As she picks up The King in Yellow, the radio turns on and a message from the "Three note oddity" numbers station is read out, while "Great holes secretly are digged where Earth's pores ought to suffice, and things have learnt to walk that ought to crawl." from H.P. Lovecraft's The Festival flashes on the screen. The broadcast ends with visions of Ariane asking Elster to "Remember our promise" and "wake up".

Elster finds herself in the Sierpinski-23 mining facility, located on the planet Leng with a photograph of a woman resembling Ariane named Alina Seo. She discovers that most of the Gestalt staff are dead and the Replika workforce has degenerated, having turned into mindless beings that attack her on sight. Elster explores the facility, gathering supplies and looking for clues about what happened and Alina's location. She opens a box belonging to Alina and encounters a "red plate that is warm to the touch", before being transported to Ariane's mother's radio station and receiving a transmitter module. She encounters the facility's Replika administrator, Adler. After a brief interaction, he pushes her down an elevator shaft after telling her "You shouldn't have returned". Elster lands on dozens of bodies from other LSTR units, surviving the fall.

Continuing her search, she finds Adler's diary, discovering he recollects memories he never lived - which he believes originated from other realities. Elster delves deeper into the mines below the facility, finding the lowest levels to consist of a gigantic mass of flesh before entering through a rectangular arch identical to the one at the start of the game. Adler despairs about the fate of Falke, the commander of the facility, who changed when she returned from beyond the arch, never explaining what she saw. Elster crosses the threshold and finds the Penrose, but tears her own arm off attempting to open the airlock and subsequently passes out, with other LSTR bodies being seen.

While unconscious, Elster dreams of Ariane, portraying a happy memory between the two during their time within the Penrose-512 in which they end up dancing to Schubert's Serenade to celebrate their 3000 cycle anniversary after embracing each other. Afterwards, Ariane is portrayed standing above Elster, asking her if she remembers their promise. Elster regains consciousness and manages to reenter the Penrose-512 from a hole on the ship, scavenging parts from another dead LSTR unit next to the cryopod to repair herself. She jumps down a hole that has appeared in the ship, finding herself back in Sierpinski-23, though the environment has become inundated with masses of flesh. Making her way through the facility, Elster drops down another hole and finds herself on the colonized moon of Rotfront. She learns that Ariane was relentlessly bullied by her classmates, signing up for the Penrose program to escape her torment. Elster continues on as the environment becomes more surreal, encountering Falke. Various collected notes reveal that Falke inherited Elster's memories and has a crisis of identity, leading her to attack.

Elster defeats Falke, as flashbacks of both Elster's and Falke's memories are shown. Elster returns to the threshold where Adler waits for her, warning Elster that no matter what she does, once she crosses the threshold, time will loop again and reality will fall apart with it shattering at any time. Elster continues regardless and a scuffle ensues, with both Replikas being mortally wounded — Adler being shot and Elster stabbed in the eye. Elster continues on, returning to the wreckage of the Penrose, where it is revealed that if the scout pilots failed to find a habitable planet, they were not expected to survive the journey. It is additionally revealed that, during the latter stages of the journey, the ship's nuclear reactor malfunctions, Ariane develops symptoms of acute radiation syndrome, and Elster seals her in the ship's cryogenic pod to keep her alive as long as possible.

Depending on the player's actions throughout the game, four endings are possible:

• To acquire the "Leave" ending, the player must be conservative with ammunition, heal frequently, and spend a long time exploring. In this ending, Elster cannot work up the courage to see Ariane again and instead leaves the ship to succumb to her wounds in the wasteland outside.
• To get the "Memory" ending, the player must utilize a balanced playstyle. Elster awakens Ariane, but she does not remember who Elster is nor their promise. Elster stays by Ariane's side until she succumbs to her wounds.
• The "Promise" ending requires the player to play aggressively, killing numerous enemies, dealing a significant amount of damage, and spending a long time in-game. Elster awakens Ariane, who remembers her and asks her to keep their promise. Elster reluctantly kills Ariane before succumbing to her own wounds.
• In the secret "Artifact" ending, after acquiring three hidden keys throughout the game (only on a second playthrough) that unlocks Ariane's safe which yields a vase of white lilies, Elster performs an esoteric ritual that results in her apparent death. A memory of her and Ariane dancing can then be seen inside the wreckage of the Penrose-512.

Accolades