Watson and the fellows spring back into action when Mary's mother accidentally poisons herself while suffering from a rare form of dementia that is progressing with impossible speed. Meanwhile, Sherlock Holmes resurfaces after being presumed dead.

When a microbiology university researcher returns from an expedition to Siberia, she and her team members fall ill from a possible revived pathogen or "zombie virus". Meanwhile, Watson catches up with his friend Sherlock Holmes after finding out he is alive, and Ingrid unexpectedly finds herself on the team's floor.