Consider the following logline, from one of my students:
LOGLINE: A wife's marriage is jeopardized when her high school boyfriend returns home full of regrets, romance and promises.
For example, the transition from act two to act three, the Second Act Plot Point, is the low point in the arc of the protagonist. The wife is the main character, and her low point could be when it looks like her marriage is over. So let's have the husband, who finds out about her "affair" with the ex-boyfriend, move out. And let's increase the stakes by having him move out with their children.
The transition from acts one to two, the Act One Plot Point, is the move from the ordinary to extra-ordinary worlds, in terms of the experience of the protagonist. The wife's ordinary world is marriage - and let's make it the habit, the routine, the "lost romance" of marriage. Her married life is in a rut, which makes her more vulnerable. Her "extra-ordinary" world then becomes a world of romantic possibilities, the world when she gives in to the fantasy of what the ex-boyfriend may offer to her. She has a new meaning in life, albeit one that is going to lead to disaster.
Let's move backwards in Act One. The call to action? When she agrees to see the ex-boyfriend. The complication? When he arrives back in town. The hook? Since this is a story about a marriage in a rut, and (the happy ending) about how the ex-boyfriend revives it, let's start with a marital argument - and let's make it really trivial, really stupid, for two reasons: to let the marriage be strong at root, just in a rut; and to show how petty the couple has become in their everyday stresses and routines.
I said we want a happy ending. Then what is Act Three? The wife saves the marriage. She realizes how much she really loves her husband and children. To make this more solid, let's make sure she does NOT have an affair with the ex-boyfriend. So we need a rumor, for the husband to believe, wrongly, that the wife has cheated on him. We can do it this way: the wife meets the ex at a motel but doesn't go through with it. But she is seen, and this is what gets back to the husband.
What about the mid-point plot point? This often is a new goal for the protagonist and here it can be, too: the wife agrees to meet the ex in a romantic setting, she gives in to his pursuit. She starts paying more attention to her looks, feeling sexy again: she gives in to the romance the ex is bringing into her life again.
So I suggest beginning with the two major plotpoints and then filling in the rest.
HOOK: wife and husband argue over something silly. Their marriage is in a rut.
COMPLICATION: wife learns that her ex-boyfriend from high school is back in town. Let's say she was a cheer leader, he the football star who went off to college and got involved with someone else, breaking her heart.
CALL TO ACTION: The ex wants to see her. She agrees. They have lunch.
ACT ONE PLOT POINT: It's not just a visit, the ex has moved back to town. And he expresses regret at breaking up with her. And he wants her back. Her world has changed.
MIDPOINT: She gives in to the romantic overtures of the ex, starts seeing him on the sly. Later she'll agree to meet him at a motel, to sleep with him.
ACT TWO PLOT POINT: She can't go through with sleeping with him but they are seen together at the motel. The news gets back to the husband, who moves out with their kids.
RESOLUTION: The wife learns how important her husband and family are to her. She confronts the ex and gets him to go with her to see the husband, where the ex tells the truth. She and her husband re-unite with a new appreciation, and a new passion, for their marriage.