It's Saturday morning at Ashton High School, and the SAT exams have just begun. As the students settle in to their seats, cracking open their exam booklets, everyone is startled by a loud crash and the wail of a car alarm. Rushing to the window, they are shocked to find the body of beloved English teacher Beth Landow strewn across the hood of a car parked at the base of a clock tower. It appears to be a suicide. But school principal Arlene Cassidy isn't so sure. That's why she calls on the services of detective Adrian Monk to investigate.
Immediately, Monk is suspicious of the details surrounding Landow's apparent suicide. Firstly, why would a woman planning to jump to her death take the time to neatly fold her jacket before climbing onto the ledge? Also, while examining the alleged suicide note, Monk discovers obvious grammatical errors that a woman who'd spent her career as an English teacher would never make. His suspicions awakened, Monk agrees to take a job at the high school as a substitute teacher to get closer the case and determine who had reason to harm the innocent English teacher.
It doesn't take long for Monk to narrow in on the most likely suspect: Science teacher Derek Philby. Monk soon learns that not only has Philby been cheating on his wife, but his mistress was none other than the late Beth Landow. But one question remains: how could Philby have been in two places at once? Philby was proctoring the SAT's when Landow plummeted from the clock tower, and has a classroom full of students to verify it. Emboldened with such an airtight alibi, Philby all but dares Monk to prove he was involved.
While once again inspecting the crime scene, the pieces of the puzzle finally click together for Monk. Philby wouldn't have had to have been in two places at once if Landow was already dead before falling from the clock tower. And how does a killer establish an airtight alibi for his whereabouts at the exact time of the victim's death? It's easy if time is on his side. Philby placed Beth Landow's already dead corpse on the horizontal arm of the clock tower, parked his car at the tower's base, then rushed off to proctor the SAT test. As the minutes ticked away, Landow's body inched closer and closer to the edge. Once the minute hand had tilted far enough down, Landow slid from the clock and tumbled onto Philby's car five stories below, setting off the alarm and alerting the campus to her suicide. It would have been a perfect murder - were it not for Adrian Monk.