Fifth-grade students, misbehaving during an air-raid drill, smash a cabinet in the physics lab. Lev Prokushev, one of the perpetrators, takes a microscope eyepiece, which is soon discovered in Arkady Zubov's school bag. Arkady categorically denies any involvement in the mischief but refuses to name the culprit.
| Actor | Role |
|---|---|
| LLoengrin MinaevArkadiy Zubov | Arkadiy Zubov |
| PPyotr GrokhovskiyGenya | Genya |
 Boris RungeLyova Prokushev | Lyova Prokushev |
| LLev MirskyVitya Surikov | Vitya Surikov |
 Dagmara DeringerLidochka | Lidochka |
 Yekaterina DerevshchikovaKatya | Katya |
| KK. GavrikovClassmate | Classmate |
| GGrigoriy AronovAronov, classmate | Aronov, classmate |
 Anatoliy IgnatyevSeryozha | Seryozha |
 Vera AltayskayaMarusya Ivanova, pioneer leader | Marusya Ivanova, pioneer leader |
 Pyotr SavinVladimir Vishnyakov, teacher | Vladimir Vishnyakov, teacher |
 Lev PotyomkinArkadiy's father | Arkadiy's father |
 Nina RusinovaArkadiy's mother | Arkadiy's mother |
 Andrei TutyshkinSchool headmaster | School headmaster |
 Elena MaksimovaVitya Surikov's mother | Vitya Surikov's mother |
| EEduard VolodarskiyClassmate | Classmate |
 Anatoli GoryunovDoctor (uncredited) | Doctor (uncredited) |
 Aleksandr Sashin-NikolskyFriend of Surikov's mother | Friend of Surikov's mother |