Five young boys find a message of a prisoner who claims to have been sentenced unjustly. The boys prepare a plan to rescue the innocent man during their summer holidays. They send a cake - with a message hidden inside - to prisoner 5823, but by mistake the cake is brought to prisoner number 6211, who is a dangerous gangster. The gangster's criminal friends plan an escape for him too and Amalia, a young singer who belongs to the gangsters, makes Gianni (Terence Hill), one of the boys, who fell in love with her, believe that the innocent man is her brother; to convince the boys to help the dangerous criminal Jack Menotti to escape. When the boys meet the gangster, they realize the deceit. The situation gets out of hand until police discovers the truth and arrives to rescue the boys. Their enterprise was not in vain though: the innocent prisoner 5823 is finally released.