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 prot?g?, 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.

Watson and the fellows race against the clock to save a man and his pregnant wife from a terrifying sinkhole. Meanwhile, Ingrid's future with her therapy group becomes unclear.

When a patient arrives at the Holmes Clinic plagued by a mysterious neurological disorder, Watson and the fellows discover they were forced to commit a harrowing, unimaginable act as a child in order to survive. Meanwhile, Adam is struggling with how his future will change as a father.

Watson and the fellows fight to revive a 30-year-old woman, Audrey, after her heart inexplicably stops repeatedly until they discover she has a rare phenomenon called Lazarus syndrome. As these revivals continue, she uses the short window of time available to right her greatest wrong.

Watson is sued for Wrongful Life by a long-term patient suffering from VACTERL association, after undergoing multiple painful surgeries.

Watson and the fellows race to save the life of a young woman who needs a lifesaving kidney donation. Meanwhile, Sherlock returns to help crack unsolved cases in Pittsburgh.

Watson investigates a case involving a mentally unstable mother, with the woman's delusions serving as a mirror to his own newly discovered hallucinations.

In the wake of the car accident, Watson and Laila lie injured and awaiting rescue as the team races to find them before it's too late. Meanwhile, in a state between life and passing, Watson's mind plays out a modern-day investigation of a case that has long haunted him.

Following his brain tumor diagnosis, Watson goes on medical leave to seek treatment, leaving Mary in charge of The Holmes Clinic as a notorious South American dictator demands Watson see to his mysterious autoimmune condition personally. Meanwhile, Ingrid grows more concerned about Beck's obsession with Sasha.

As Adam's wife goes in for a C-section to deliver triplets, chaos ensues when their OB-GYN passes out in the operating room. Meanwhile, a severe storm detours Watson and Mary's trip to find a renowned doctor to remove Watson's brain tumor.

A seemingly random cuddle outside of UHOP sets the stage for the arrival of someone from Watson and Sherlock Holmes' past.