Watson and the fellow 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. Sherlock Holmes resurfaces after being presumed dead.

Having been presumed dead, Watson is shocked to discover Sherlock Holmes in his apartment, and recounts a recent case involving a microbiology team being afflicted with a "zombie virus".

A newly reinstated Ingrid helps the team treat a 30-year old patient who appears to be only 10.

Watson and team treat 4 patients with suspected flesh-eating bacteria after tornado. Shinwell seeks purpose in nursing school while Adam's wedding to Lauren faces obstacles.

While visiting another hospital, Watson becomes convinced that a man is suffering from locked-in syndrome and has been mistaken for a coma patient for over a decade; Sherlock is back in town and is staying with Watson.

The team rush to save a young athlete with an apparently untreatable tumour. Further troubles arise when Mycroft Holmes throws the clinic's future into doubt.