Shang-Chi and the Legend of the Ten Rings may hide a dark secret about Katy, but the Marvel Cinematic Universe has a reason for holding it back.

What if Shang-Chi and the Legend of the Ten Rings hides a dark secret about Katy, and who Shang-Chi murdered on his mission to America? In Shang-Chi, Simu Liu's titular character hides in San Francisco from his father, Wenwu (Tony Leung), and his life as a trained assassin, but one compelling Marvel theory suggests that there's more to that hidden past that could make Shang-Chi more like Sebastian Stan's Winter Soldier. In fact, it flips Shang-Chi and Katy's relationship upside down and would have huge ramifications in the future if it proved to be true.

Ying Li (Fala Chen) is one of Shang-Chi's most important characters: not only is she the narrative driver behind Wenwu's ill-fated attempt to open the portal to the dark realms, but of course, her death is the catalyst for Wenwu taking up his magical Ten Rings again and training his son to become an assassin. When Shang-Chi is a teenager, his father tasks him with killing the man responsible for his mother's death. Though Shang-Chi finishes the assignment, he can't continue to be a killer, so instead, he leaves his old life for San Francisco and hides until his father sends his assassins a decade later. He carries the burden of that killer assignment, still, only revealing that he went through with it late in Shang-Chi's story.

There may be something dark about Shang-Chi's past that the movie hasn't revealed yet. In the film, Katy and Shang-Chi became friends after he defected from the Ten Rings, but there may be something sinister to their relationship. According to a theory (via TikTok), Shang-Chi killed the grandfather of Awkwafina's Katy Chen, and Marvel just hasn't revealed it yet. According to the theory, the moment could've been cut from the film because the storyline was too close to Bucky's (Sebastian Stan) story from The Falcon and the Winter Soldier since he befriends the father of one of his victims.

Shang-Chi provides several plot points that support the idea. If Shang-Chi did kill Katy's grandfather, it'd explain why he was in San Francisco to start his new life, and it'd explain how he and Katy crossed paths in the first place. He felt guilty about the death, and eventually, the pair formed a friendship. Also, later in the film, Shang-Chi admits to Katy that he's not the person she thinks he is, saying that he killed his target before leaving the dangerous Ten Rings organization. (He previously told her he didn't commit the murder.) The scene could've easily fit in a moment where Shang-Chi admitted he killed her grandfather. After all, Katy wears a look that's incredibly distressed throughout the conversation, hinting that perhaps the line existed at some point.

Thus far, Shang-Chi and the Legend of the Ten Rings neither confirms nor denies the grandfather theory. While there's enough evidence to suggest that Shang-Chi and Katy have a deeper connection than the film reveals, there's no definitive answer either way. However, with Falcon and the Winter Soldier treading similar ground, it makes sense that the MCU film would drop the plot point for now. But, of course, Shang-Chi will be a part of the MCU for years, so Marvel has plenty of time to reveal the potential twist.

More: Shang-Chi's Ten Rings Can Make Him Fit The Avengers' Most Powerful Team