At the beginning of A Wrinkle in Time, Meg Murry is not at all confident of her abilities in general. She does not feel intelligent; her teachers make it clear they don't think she is very bright. Meg compares her looks to her mother's, and is not happy in that area, either. She knows, though, that she has a special link to her younger brother, Charles Wallace. Their father, a physicist, is missing, and although they and their friend Calvin go to some strange places to try to rescue him, Meg does not have any confidence that she has any ability to save him. She believes that it will be their father that saves them and defeats the evil that has him prisoner.
When the evil engulfs Charles Wallace and almost traps the three children and their father, Meg is almost killed. She realizes that her father is not the one who can save them, and that she is the one with the connection to Charles Wallace that can defeat the evil. Meg will not give up, and this stubbornness is yet another supposed flaw that helps her. She finally understands that while she has been trying to defeat the evil with more hate, that neither that nor intelligence is what can defeat the evil. It is what she has, and the evil does not, that defeats it--the power of love. This is Meg's strength, and with it she saves them all.