One of his customers is self-styled 'Colonel' Caplan, who operates a military-style executive survival programme from his country estate.
These suspicions appear justified when a consignment of CS gas is found in the warehouse, Arthur claiming not to know how it got there.
Investigating further, Arthur, Terry, DS Rycott, DC Jones and Angie are held captive at the estate whilst the robbery goes down.
But thanks to the defective equipment supplied by Arthur, it fails and the police manage to get there in time to nab the thieves.
A confrontation with senior police leads to Arthur being stripped of his business interests (and possibly arrest), Chisholm close to a second nervous breakdown, and Rycott and Jones getting a major roasting.