Penumbra: Black Plague is a Swedish adventure game with a focus on first-person exploration with a horror theme. There is a heavy emphasis on using physics to solve many of the game's puzzles. Unlike the previous game, Penumbra: Overture, it requires that players avoid direct confrontation entirely as no weapons are available during the course of the game.
Black Plague picks up exactly where Overture left off, with the protagonist, Philip, waking up in a cell after being knocked unconscious. Upon escaping, Philip must find clues that hint to his father, Howard, still being somewhere inside the underground facility. However, he soon realises that he is not alone and that he must avoid the infected creatures that inhabit the underground laboratory. He must also try to ignore the voices inside his mind, the various traps and the vile infected.
Black Plague was originally envisioned as the second part of a trilogy of games, however upon release, it was announced to be the ending of the storyline due to publisher troubles. However, an expansion, Requiem, was released six months later.
Initially the game starts with Philip sending an email to a friend telling him to finish the job he started and explaining his situation. He wakes up in a cell after being knocked unconscious at the end of the previous game and manages to escape from the prison only to find himself inside some underground facility. It turns out the facility belongs to a somewhat secretive and elusive organisation, the Archaic Elevated Caste, who are committed to researching and finding ancient knowledge and things related to it.
It appears the facility has long since been abandoned with the staff having either been turned into the infected who are zombie-like creatures or simply dead. However, in a strange turn of events it appears that Philip has also contracted the virus which seems to turn others into the infected creatures. Philip, though, does not seem to have joined with the hive-mind but instead developed a malevolent split personality going by the name of Clarence. It's this voice in his head that continually taunts and ridicules him throughout the game.
Upon further exploration of the facility Philip comes into contact, over the communication network, with a woman by the name of Amabel Swanson who was a member of the Archaic researchers; she offers to help Philip, as long as he can make his way to the other side of the facility where she is located. As Philip makes his way there he comes across various pieces of information which explain what the Archaic were doing there. It turns out they were trying to locate an ancient entity called the 'Tuurngait'. Upon finding and releasing it, the entity transformed itself into the virus that had turned the personnel in the base into infected creatures. Further investigation sheds light on the fate that befell Philip's father, Howard. It appears that Howard managed to make contact with the Tuurngait, but he committed suicide before sending his letter to Philip begging him to destroy his research documents and all knowledge he had obtained.
After finding Amabel and having to put up with Clarence and his madness, Philip hallucinates and accidentally kills Amabel. Looking through her data and using a specialised machine in her lab, Philip manages to remove his other personality, Clarence, for good.
However, this causes Clarence to be transported into a corpse who then proceeds to attack Philip, but the risen corpse is soon destroyed by a group of infected creatures who have been sent to save Philip by the Tuurngait itself. The mysterious entity then proceeds to speak to Philip through his mind and puts him through a number of spiritual tests to test his ability to be a part of the hive-mind that the Tuurngait seems to use to control those infected by its virus. Upon completing the test, the Tuurngait reveals the full story.
It seems that many millions of years ago the Tuurngait arrived on the Earth and lived alongside the native Inuit; it infected them with a virus to pass on its vast knowledge. As humanity began to evolve and expand, the Tuurngait severed its link with humanity and burrowed down into the earth where it went into a form of hibernation. It's there that the Archaic organisation located the entity and it struck out in self defense due to being disturbed and threatened. It realised that humans as individuals were to be commended, but as a whole they were failures due to their destructive and selfish natures.
However, it felt something different about Philip and leaves its fate in his hands, but insists he contact someone above ground to destroy the laboratory that Archaic constructed in order to allow the Tuurngait to continue its slumber. It's then revealed that the entity made the same request to Howard, but instead of doing as his father asked, Philip came to investigate. This is where the game links back to the beginning, where we see Philip sending an email to his friend explaining the situation and the request by the Tuurngait.
Philip, at the last minute, decides to follow his own path instead and comments that he has more in common with his Clarence than the Tuurngait. As a result, at the end of the email he tells his friend the location of the Tuurngait and asks that his friend come and destroy both it and the remaining infected.