To fall into someone's trap or to have things turn out exactly as someone wished; to act according to another person's plans (often used negatively).
彼の褒め言葉に簡単に乗って、思う壺にはまってしまった。
I was easily taken in by his compliments and played right into his hands.
最初から相手の作戦だと気づいていれば、思う壺にはまらなかったのに。
If we had realized from the start that it was the opponent's strategy, we wouldn't have fallen into their trap.
交渉で相手の思う壺にはまらないように注意した。
I was careful not to play into the other party's hands during negotiations.