An absent minded professor invents a pen that will explode on the sound of bells, then leaves it in a taxi.
An absent-minded professor has developed a booby-trap device hidden inside a fountain pen. He demonstrates it to a group of soldiers at an army base. It is filled with high explosive and will detonate at the sound on Big Ben chiming on the hour, although only when the pen cap is removed. He later looses the pen inside a taxi and discovers that it was found by a nightclub barman who has since sold it to a pawnbroker.
With the prospect of £30 reward for the pen's return the barman spends a long time searching for the device which has since been bought by someone, and is then stolen again, only to find that it was the wrong pen. Meanwhile the professor has stumbled on a dope-peddling racket being operated by the nightclub owner at the club-cafe where the barman works. Addicts buy a pawn ticket from the owner and pick up their dope at the pawn shop (the dope is secreted inside fountain pens).
Eventually the police are called in but the club owner and his henchman have made a getaway. However the professor points out that the club owner still has the booby trapped pen in his possesion thinking it was one of his drug-filled pens. Their car radio plays the news at midnight and the chimes of Big Ben cause the pens to explode killing both of them.
