Common Vector is the third episode of Series 26 and the 784th episode of Casualty overall.
New mum Annie, skateboarder Richie, white-collar worker Chris and protected teenager Fiona are all admitted with sickness and dilated pupils. Observant doctor Dylan soon notices a pattern but what is the common link? With the patients deteriorating, the race is on to get to the bottom of the mystery. Could the connection be a poisoned supply of heroin which has found its way on to the streets? And will the team get to all the patients in time – including breastfed baby Evie?