Simon James, portrayed by Jesse Eisenberg, is a young Canadian man navigating life in London, England. His chaotic day begins when, after exiting a tube train, his briefcase gets caught in the door, leading to the unfortunate loss of both his bag and ID badge, which he desperately needs for work. Upon arriving at the office, Simon attempts to persuade his boss, Wallace Shawn, to consider his ideas for the business. To complicate matters further, Simon learns he is set to tutor his boss's daughter, Melanie, played by Yasmin Paige.
During a visit to a nursing home to see his mother, Phyllis Somerville, Simon meets another resident who ominously suggests that he is "not right." Back in his apartment, Simon develops an obsession with his attractive co-worker, Hannah, portrayed by Mia Wasikowska. He watches her through a telescope and discovers her in the process of destroying her artwork. When he retrieves the torn pieces and brings them back to his home, he finds solace in reassembling her work.
One fateful night, Simon witnesses a man leap from a ledge above Hannah's apartment, watching helplessly as he falls to his death. After discussing the tragedy with police detectives, he learns that a slight change in position could have saved the man's life.
In an attempt to connect with Hannah, Simon takes her out to dinner, but his plans get interrupted by a call from his mother. When he returns, he finds Hannah has left a note promising to see him later. In a bid to impress her, Simon decides to trade in his television for cash to buy her a gift.
While at work, Simon encounters a doppelgänger - James Simon, who is also played by Jesse Eisenberg. Unfortunately, nobody else seems to notice their striking resemblance. As Simon faints from the shock, he eventually finds himself spending more time with James, who becomes increasingly popular at work and catches Hannah's eye. On a date with her, Simon pretends to be James, with James feeding him lines through an earpiece. This charade culminates in disaster when James kisses Hannah, deepening Simon's feelings of betrayal.
Matters spiral further when Simon finds himself cornered by James, who skillfully blackmails him to gain access to his apartment. Tension mounts when Hannah becomes obsessed with spying on James to see if he is unfaithful. When Simon confronts James, he is violently attacked, leading to a breaking point in his sanity. His emotional outburst at work sees him fired, driving Simon to a dark place, where he contemplates suicide.
In a twist of fate, he discovers that Hannah has overdosed and brings her to the hospital, where he learns of her tragic miscarriage from a liaison with James. As Simon grapples with the loss of his mother and the reality of his deteriorating situation, he confronts James during her funeral. This confrontation results in a brutal realization about their mirrored existence.
Pushed to the brink, Simon handcuffs James and takes a leap from a ledge intending to end it all. In a stroke of irony, he survives the fall with severe injuries while Hannah rushes to his side. Amid the chaos, James meets his demise, and Simon, even in pain, replies with a newfound clarity: “I'd like to think I'm pretty unique.” In a journey of identity and self-discovery, Simon emerges from the shadows, hoping to finally be seen and acknowledged.
