While on their vacation in Matera, Madeleine Swann constantly alludes to her secrets that she intends to tell James Bond. While Bond is ensuring her departure on a train, Swann's hand noticeably goes to her belly right before the doors close, indicating a desire to tell him about her pregnancy but losing the immediate opportunity to do so.
Bond first meets Mathilde when visiting Madeleine at her childhood home on the frozen lake. Despite the girl obviously having similar features to Bond, she tells him that she is not his.
Mathilde later becomes caught up in the Project Heracles conspiracy when Bond and Mathilde are forced to flee hurriedly from their enemies. They escape into a forest, but despite Bond's best efforts, Mathilde and Swann are captured by Lyutsifer Safin, the ringleader of the conspiracy. Safin takes the two of them to his base of operations, an old World War II base, and introduces Mathilde to his garden of poisonous plants. He then separates the two, holding Mathilde as a hostage and claiming that he will raise her as his own.
Although James Bond eventually reaches Safin, he finds himself seemingly at Safin's mercy. Safin forces Bond to surrender his weapons, threatening Mathilde's life if he does not comply. Ultimately, Bond gets the jump on Safin with a hidden weapon, but Safin uses a secret trapdoor to flee with Mathilde. Along the way, Mathilde loses her favorite toy. When Safin ignores her pleas to go back for it, she bites his hands and he abandons her as being more trouble than she's worth. She hides under a table and is soon discovered by Bond and Swann, who have reunited.
Swann and Mathilde flee to safety, while Bond stays behind to ensure that Project Heracles is destroyed once and for all. He notices Mathilde's stuffed toy lying on the ground and puts it into his belt. He then realizes that the base's doors are closing yet again, forcing him to make his way back to the control room. Safin then attacks him, smashing the vial with nanobots coded with Madeleine's DNA upon Bond's face. Bond kills Safin in return, then calls Madeleine, who confirms that Mathilde is indeed is daughter. Unfortunately, Safin's attack means that he can never see either of them again. He remains behind as the base is destroyed by missiles and is killed. Afterwards, Madeleine tells Mathilde the truth of her parentage.
- Mathilde's presence in the film was kept a strict secret prior to its premiere and the character did not appear in any trailers or publicity footage. A behind-the-scenes shot of the character has her face pixelated.
- Mathilde is half-French, quarter-Swiss-French and quarter-Scottish. She would be entitled to British, French and Swiss citizenship.
- This is not the first time Bond has had a daughter. In the unrelated, non-Eon film the 1967 Casino Royale, Bond has a daughter with Mata Hari called Mata Bond. Though there is some similarity between the names Mata and Mathilde, this appears to be coincidental.