Puppet On A String is an episode of series 20 that first aired on 6 January 2005.
The team are called to a hospital where a doctor has been attacked by an assailiant carrying a lethal syringe of drugs. The doctor is adamant that one of his colleagues is responsible, but the man has a cast-iron alibi, and it becomes clear that the two men are arch-rivals. The situation is further complicated when the victim's wife is later seen having a conversation with a known hitman. Is one of them the killer?