In a flat in London, Ronnie and Sam are investigating an explosion which killed a man, Jonathan. They discover that he was killed when a package which came in the post exploded. At first, Ronnie and Sam are convinced that it is a hate crime, searching out a list of potential suspects that could have had a hand in his murder. But as they try to narrow down the list of who could be responsible, they start to find themselves at a dead end.

At the lab, they discover that the bomb contained some banned substances, such as mercury. Following leads, they trace Jonathan’s history to a university where he used to be a cleaner. Ronnie and Sam then discover that student Simon Wells could be responsible – with a vendetta against the dead man.

During questioning, Simon reveals that it was indeed he who killed Jonathan, because he listened to the voices in his head. It seems like a clear cut case of schizophrenia, but when the investigation digs deeper, it transpires that Simon’s doctor, Martin Middlebrook, was conducting an investigation into a new type of schizophrenic drug which he was testing on Simon.

But as Alesha investigates, she uncovers a PET scan of Simon’s brain that was taken by Dr Middlebrook. Simon has a brain tumour, which altered his personality and made him send the bomb in the post to Jonathan. Martin Middlebrook deliberately failed to address his health problems for his own gain, and Jake tells him that they’ll be charging him with manslaughter, gross negligence, and when the time comes, murder, too.