Cars are being stolen in worryingly high numbers, and with the numbers recovered falling dramatically, the signs point to a crooked cop somewhere in the department. Mark is convinced that the cop in question is innocent, but nobody else seems very sure.

An old friend of the Chief's tries to commit suicide, after local gossip and the media both blame her for the murder of her husband. Ironside is determined to find out the truth about the death, even if it means proving that his friend really is guilty.

A string of murders is committed, and in each case the victim has a numbered disk in their possession. After number four is found dead, it transpires that the Commissioner is the owner of disk number six; and the Chief disguises himself as number five - an irascible artist to whom he bears a striking resemblance - in order to keep him alive.