Fletcher Reede is a lawyer who will do anything to win court cases. Including telling lies. On the night of his son's fifth birthday, he is busy sleeping with his boss instead of being at the party. As five year old Max blows out the candles on his cake, he makes a wish that for only one day, his father would be able to tell the truth, the whole truth and nothing but the truth. The wish comes true.
Fletcher is handling a divorce case where the prenuptial agreement states that as a result of the wife's infidelity, she is entitled to nothing. He was planning to manipulate the truth in order to win but the inability to lie renders this impossible.
At the same time, Fletcher's ex-wife, Audrey, is planning to move to the other side of the US with her boyfriend, taking Max with her. In court, Fletcher realises that his client was a minor when she got married, making the prenup void so she wins half the marital assets. He speeds off to the airport to catch his son, but Max is already on the plane which is on the runway preparing to take off. Hijacking a set of mobile stairs, he keeps the plane on the ground breaking both legs and being thrown in jail. A year later, the guest list at Max's 6th birthday party is select - just his parents. He blows out the candles, plunging the room into darkness. When Max turns the light on, Fletcher and Audrey are kissing. The film ends as it pans out from the house where Fletcher is chasing Max trying to tickle him.