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.

Watson and the team work alongside Laila to help a patient named Maxine, who is 30 years old but looks 10 years old. Meanwhile, Watson convinces Laila that he is serious about their relationship and Ingrid is struggling to ingratiate herself back at work as a hot newcomer joins her therapy group.

Watson and the team jump into action when four new patients are brought to UHOP with wounds that appear to be infected with a flesh-eating bacterium following a tornado. Meanwhile, Shinwell searches for meaning as a nursing student and Adam's wedding to Lauren risks being halted.

While visiting another hospital, Watson becomes convinced that a coma patient is suffering from locked-in syndrome and has been mistaken for a coma patient for over a decade. After the patient is transferred to UHOP for treatment, he shocks the team with chilling messages. Meanwhile, Sherlock is back in town and is staying with Watson.

Watson and the fellows race against time to save the life of Xavier, a 21-year-old athlete with a fast-growing, seemingly incurable cancer. Meanwhile, Mycroft Holmes lets Watson know his team's work at UHOP may no longer have funding available.

Watson's father, Hamish, visits UHOP to introduce Watson and the fellows to his protégé, Annabelle, a saxophone prodigy with musically induced blackouts. As Annabelle's condition worsens, Watson must confront his long-standing tensions with his father in order to save her.

UHOP veers into terror when Watson and the fellows are taken hostage by a man with a bomb strapped to his chest who is desperate to find a cure for his 9-year-old daughter's illness.

Watson and the fellows help Laila's son Micah - who was injured in a car accident while pursuing his crush, Bex, with the help of his AI chatbot.

Watson's biohacker friend Hobie returns when he rushes his new girlfriend to the Holmes Clinic after they partake in a gene-editing experiment that goes wrong. Meanwhile, Watson becomes suspicious of Sherlock.