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; Watson becomes suspicious of Sherlock.

Watson and the fellows help Laila's son Micah, who was injured in a car accident while pursuing his crush.

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's father, Hamish, visits UHOP to introduce Watson and the fellows to his protégé, Annabelle, a saxophone prodigy with musically induced blackouts.

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; Mycroft Holmes lets Watson know his team's work at UHOP may no longer have funding available.

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.

Watson and the team jump into action when four people become infected with a flesh-eating bacterium; Shinwell searches for meaning as a nursing student; Adam's wedding to Lauren risks being halted.

Watson and the team work alongside Laila to help a patient named Maxine, a 30-year-old who looks 10.

When a microbiology university researcher returns from an expedition to Siberia, she and her team members fall ill from a possible revived pathogen; Watson catches up with his friend Sherlock Holmes after finding out he is alive.

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; Sherlock Holmes resurfaces after being presumed dead.