Cobb's wife Mal (played by Marion Cotillard) kills herself prior to the events of Inception - but what exactly prompted her to take her own life?

Cobb's wife Mal (played by Marion Cotillard) kills herself prior to the events of Inception after coming to believe what she thought was reality is really just another layer of dreaming. Christopher Nolan's sci-fi movie leaves viewers constantly wondering whether what they're seeing is a dream, a dream within a dream (sometimes within another dream), or the film's version of reality... assuming that's not just a dream in the head of its protagonist, Dom Cobb (Leonardo DiCaprio). With concerns like that, it's little wonder Cobb frequently checks to see if he's unknowingly still dreaming and appears ready to wake up by "killing" himself, should he suspect otherwise.

But how does one determine if they're dreaming without realizing it? As Inception explains, Cobb and his team have personal items they call totems. These seemingly everyday objects (like a top or a die) have been altered or modified, but only in a way their owner knows (for example, Joseph Gordon-Levitt's Arthur has a loaded die). That way, if they want to test whether they're in a dream or not, they merely need to check their totem and see if it "feels" right and/or abides by the laws of physics as only they know it should. For this reason, it's imperative the owner be the only person to ever touch their totem; otherwise, they can't trust they're not in the dream of someone else who knows how their totem has been modified and can change the dream in order to trick them.

This all comes back into play during a pivotal scene where Cobb explains to his team's newest member, Ariadne (Ellen Page), why Mal killed herself before the beginning of Inception. He and Mal had been experimenting with dream-sharing technology when they found their way into Limbo (a term that refers to infinite subconscious). There, Mal became increasingly obsessed with the power of being able to shape their dream world and refused to return to the real one, prompting Cobb to perform a rudimentary version of Inception (planting an idea in someone's subconscious mind) by reactivating her totem. And while he succeeded in planting a simple idea - that her world isn't real - in her head, it continued to affect her even after they woke up, and not in a helpful way, either.

Upon returning to reality, Mal gradually became convinced what she thought was the real world is really just another dream and tried to convince Cobb they should "wake" themselves up by killing themselves. When he refused, she committed suicide and framed Cobb for her death in an attempt to get him to do the same. After that, Cobb was continuously haunted by his own subconscious' projection of Mal, who constantly sabotaged his attempts to infiltrate the dreams of others and reminded him of his own doubts about whether reality is, well, real. She - or, rather, the manifestation of Cobb's guilt about his role in causing Mal's death - isn't necessarily wrong, either. Even by the movie's (infamously ambiguous) final scene, neither he nor the audience know for certain if the film's version of reality is truly "real", not least of all because the totem he now uses is the same one Mal did before her death.

That being said, Inception seems to argue it doesn't matter; Cobb knows what's important to him (being reunited with his children), and he will simply live out his life and wait to see what comes next. He might even discover Mal was right and she's been waiting for him in the real-world for a few hours since she "killed" herself (what with time working differently in dreams versus reality). Whatever the case might be, she's a cautionary tale that illustrates what Dom means when he talks about the power of ideas (comparing them to a "highly contagious" virus that can "grow to define or destroy you") and why Inception isn't a process to be carried out casually on anyone.

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