At the beginning of the play, Leonato, the Governor of Messina, receives a letter to the effect that Don Pedro has been victorious in battle, and...
is returning home to Messina. Seemingly out of the blue, Beatrice asks about Benedick, identifying him obliquely and derogatorily:
BEATRICE. I pray you, is Signior Mountanto returned from the wars or no? (1.1.25-26)
Beatrice piles insults on Benedick for the next fifty lines, and when Benedick appears in the scene, she continues to insult him to his face in a verbal exchange similar to, but considerably shorter than the exchange between Petruchio and Katherine in The Taming of the Shrew, which Shakespeare wrote two years before he wrote Much Ado About Nothing.
Clearly, Beatrice and Benedick are in love. Equally clear is that they have great difficulty expressing their love to each other.
At the masked party in act 2, scene 1, Beatrice and Benedick are dancing together, and both are masked, although Beatrice knows quite well she's dancing with Benedick, who thinks he fooling Beatrice with his disguise. Beatrice once again takes the opportunity to rail against Benedick:
BENEDICK. I pray you, what is he?
BEATRICE. Why, he is the prince's jester, a very dull fool. Only his gift is in devising impossible slanders. None butlibertines delight in him; and the commendation is not in his wit, but in his villainy; for he both pleases men andangers them, and then they laugh at him and beat him. Iam sure he is in the fleet. (2.1.118-123)
Later in that same scene, Beatrice provides a clue to the reason for this unceasing animosity between Benedick and herself:
DON PEDRO. Come, lady, come; you have lost the heart of Signior Benedick.
BEATRICE. Indeed, my lord, he lent it me awhile, and I gave him use for it—a double heart for his single one. Marry, once before he won it of me with false dice; therefore your Grace may well say I have lost it. (2.1.243-248)
Apparently, their confession of love to each other in act 4, scene 1 isn't the first time that they've expressed such feelings, although their previous romantic experience seems not to have gone very well.
The scheming of Benedick's friends in act 2, scene 3, and the scheming of Beatrice's friends in act 3, scene 1 to make Benedick and Beatrice face up to their true feelings for each other, helps prepare them for their declarations of love in act 4, scene 1.
Act 4, scene 1 begins with what is supposed the be the marriage of Hero and Claudio, but the scene takes a sharp turn in a different direction.
Claudio accuses Hero of being unfaithful to him, Hero faints, Hero's father is apoplectic, and Claudio marches out of the church in self-righteous indignation, leaving Hero in what appears to be a dead faint at the foot of the altar.
For his part, Benedick seems confused about the whole marriage business:
BENEDICK. This looks not like a nuptial. (4.1.67)
Fortunately, Hero recovers from her fainting spell. The Friar who was supposed to have performed the ceremony devises a plan whereby word will be given out that Hero is dead, to give Hero's friends time to discover the truth of the matter. (What is it with Shakespeare's friars who are forever devising plans that include the heroine of the play appearing to be dead?)
Beatrice is in utter distress at the unfortunate turn of events, and she's enraged at Claudio for his unforgivable treatment of her cousin, Hero.
Benedick feels sorry for Beatrice, and tries to comfort her. Benedick offers to do whatever it takes to make Beatrice feel better, and after yet another insult to his face, declares his love for her:
BEATRICE. Ah, how much might the man deserve of me that would right her!
BENEDICK. Is there any way to show such friendship?
BEATRICE. A very even way, but no such friend.
BENEDICK. May a man do it?
BEATRICE. It is a man's office, but not yours.
BENEDICK. I do love nothing in the world so well as you. (4.1.271-277)
Beatrice is momentarily caught off guard—as is the audience at this unexpected turn of events in the middle of all of this marriage drama—and declares her love for Benedick. She tries to take it back, but eventually confirms her feelings for him.
Benedick is caught up in the moment, and, eager to prove his love, makes a fateful request:
BENEDICK. Come, bid me do anything for thee. (4.1.297)
Beatrice's response to Benedick's heartfelt but naive request is equally fateful:
BEATRICE. Kill Claudio. (4.1.298)
Benedick is taken aback:
BENEDICK. Ha! Not for the wide world! (4.1.299)
This is not what Beatrice wants to hear. She rants and raves against Claudio, wishes she was a man so she can eat Claudio's heart in the market place, insults Benedick's manhood, rejects his love, and resolves to "die a woman with grieving" (4.1.330).
Benedick gives in:
BENEDICK. Enough, I am engaged, I will challenge him. I will kiss your hand, and so I leave you. By this hand, Claudio shall render me a dear account. (4.1.336-338)
Beatrice doesn't say anything further in the scene, but she's no doubt pleased that Benedick has chosen to prove his love for her by agreeing to avenge Claudio's reprehensible behavior towards Hero on Beatrice's behalf.