Leonard (Guy Pearce) suffers from a rare brain disorder – the inability to form any new memories. He can remember in detail everything that happened before his injury, but anyone he has met or anything he has done since that fateful night simply vanishes. Who are his friends? Who are his enemies? What is the truth? In Leonard's world, the answers to these questions shift and change from second to second. And the more he tries to figure out what is true and real, the deeper he sinks into a multi-layered abyss of uncertainty and surprises.