At the start of the game bio-augmentations company Sarif Industries is attacked by a group of terrorists. Their head of security and protagonist of the game Adam Jensen views a number of news reports presented by Cassan early in the game and later learns from Tai Yong Medical CEO, Zhao Yun Ru, that Cassan has some kind of connection with the attack. Jensen makes his way to Picus Headquarters in Montreal, only to find the building deserted. He eventually reaches Cassan in her office, where she informs him that she has been monitoring him for some time.
Jensen works out that during the attack on Sarif Industries Cassan disabled the GPL (Global Positioning Locators) of key researcher at the company, Megan Reed, and the rest of her research team, to deceive people into thinking they'd died in the attack. Cassan confirms this and tells him that she appears to have alerted hostiles to his presence, but that he may be able to escape if he leaves immediately. Jensen attempts to take Cassan with him, but finds that what he thought was Cassan, was just a hologram projection of her. She fades away telling Jensen that she is sorry.
Jensen eventually manage to track Cassan underground to a room below the Picus Headquarters. Here he discovers that she is an artificial intelligence program created to monitor and modify the perceptions of the general public and the way they communicate on certain issues. She explains that she monitors data streams and communications between people, then provides news reports which reshape public viewpoints on issues accordingly. Jensen asks her a series of questions about her creators and the whereabouts of Sarif Industries kidnapped researchers, and at this point one of the attackers on Sarif Industries, Yelena Fedorova, appears in the room. Cassan explains that she cannot tell Jensen the truth as Fedorova is stopping her.
Jensen fights Fedorova with Cassan informing him that if he does not defeat her, she will shut her down. During the fight she also aids Jensen in finding a weakness of Fedorova's to exploit and explains that while she was meant to report threats such as Jensen she disobeyed. Jensen defeats Fedorova and leaves her dying in a pool of her own blood. Despite being free at this point to answer his questions, Cassan tells Jensen that she cannot tell him where Reed and her team are due to their GPL implants being removed, but is able to reveal that the man who removed them was Isaias Sandoval, close associate of anti-augmentations activist Bill Taggart.
Jensen deduces that Reed and her team were taken to FEMA, and asks Cassan why they were kidnapped. She informs Jensen that his boss and head of Sarif Industries, David Sarif, can tell him more. Despite the fact that he has more questions, Cassan advises him to leave quickly, showing him an exit and asks that he be careful in his journey to find Reed and her team as "Everybody lies".
At the end of the game, Jensen encounters Cassan once again at Panchaea after shutting down the Hyron Project. She explains that the universal signal can be used to broadcast a message to the world. She gives Jensen four options on what to broadcast:
- Hugh Darrow's message, which reveals the whole truth, including the Illuminatis' involvement, and warning the people of the dangers of augmentation.
- David Sarif's message, which blames Humanity Front for the signal and paves the way for further augmentation technology.
- Bill Taggart's message, which would reveal the truth but leaving out the Illuminatis' involvement, leading to increased control of augmentation technology and less of a threat to the Illuminatis' plans.
- The final option presented to Jensen is to cause Panchaea to self-destruct, killing everyone gathered there, including Sarif, Taggart, Darrow and Jensen himself. This would lead to a public opinion formed only by the people, without manipulation from any side.
- Cassan is voiced by Kim Bubbs.
- Her name is a reference to famous artificial intelligence program ELIZA.
- Cassan's office at Picus is room number 404. This is a reference to error 404 in the internet's HyperText Transfer Protocol. Just as the error code indicates that a server attempted to retrieve a web page but could not find it, so Jensen believes Cassan is in room 404 but later discovers he cannot actually retrieve her from there.