模範高材生布朗溫恩、校花艾蒂、問題學生納特、棒球好手庫柏以及校園八卦app的創辦人賽門五個海景高中的學生，因為違反禁止帶手機的規定，全都被艾弗瑞老師召集到悔過室。
然而布朗溫恩很清楚那根本不是她的手機，一定是別人的惡作劇不論是開玩笑、還是為了陷害她，布朗溫恩拿出真正屬於她的手機給艾弗瑞老師看，想說明這才是她的手機，而且平常都放在置物櫃裡沒有拿進課堂教室，沒想到艾蒂、庫柏和賽特也異口同聲地說他們也是，當矛頭指向因為毒品前科出入悔過室已成家常便飯的納特時，艾弗瑞老師顯然對他們的辯稱並不買帳，並要求每個人必須寫完五百字的悔過書才能走出教室。
手機謎團還沒結束，事情便一件件接連發生，當天賽門並沒有順利走出悔過室，他用塑膠杯喝下一口水後產生劇烈的過敏反應，昏厥並離奇死亡，警方介入調查後還發現賽門的死似乎不單純，賽門死前已準備好隔天要公諸於世的發文，是有關另外四人布朗溫恩、艾蒂、納特與庫柏的醜聞爆料。
The Breakfast Club meets Pretty Little Liars, One of Us Is Lying is the story of what happens when five strangers walk into detention and only four walk out alive. Everyone is a suspect, and everyone has something to hide.
Pay close attention and you might solve this.
On Monday afternoon, five students at Bayview High walk into detention.
Bronwyn, the brain, is Yale-bound and never breaks a rule.
Addy, the beauty, is the picture-perfect homecoming princess.
Nate, the criminal, is already on probation for dealing.
Cooper, the athlete, is the all-star baseball pitcher.
And Simon, the outcast, is the creator of Bayview High s notorious gossip app.
Only, Simon never makes it out of that classroom. Before the end of detention Simon's dead. And according to investigators, his death wasn t an accident. On Monday, he died. But on Tuesday, he d planned to post juicy reveals about all four of his high-profile classmates, which makes all four of them suspects in his murder. Or are they the perfect patsies for a killer who s still on the loose?
Everyone has secrets, right? What really matters is how far you would go to protect them.
