Initially, Mary testifies against Abigail Williams and the other girls, accusing them of falsely accusing innocent people of witchcraft. However, later in the play, under Abigail's influence, Mary recants her testimony and claims that she falsely accused them when, in fact, it was Abigail who told her what to say. Mary's fear of Abigail and desire to be part of her powerful group drive her to lie and support Abigail's false accusations, leading to the play's tragic consequences.