VIDEO: Sneak Peek - Rhys Ifan Guests on Tonight's Season Finale of ELEMENTARY

On tonight's ELEMENTARY titled 'The Grand Experiment,' tising tension between Sherlock and Watson bring their partnership to a crossroads, but they endeavor to put their differences aside while they help Sherlock's brother, Mycroft, who faces accusations of treason and murder.

Tonight's season finale of ELEMENTARY airs (10:01-11:00 PM, ET/PT) on the CBS Television Network. Rhys Ifans returns as Mycroft Holmes. Get a sneak peek below!

ELEMENTARY stars Jonny Lee Miller as detective Sherlock Holmes and Lucy Liu as Dr. Joan Watson in a modern-day drama about a crime-solving duo that cracks the NYPD's most impossible cases. Following his fall from grace in London and a stint in rehab, eccentric Sherlock escaped to Manhattan where his wealthy father forced him to live with his worst nightmare – a sober companion, Dr. Watson. A successful surgeon until she lost a patient and her license three years earlier, Watson viewed her new job as another opportunity to help people, as well as paying a penance. However, the restless Sherlock was nothing like her previous clients. He informed her that none of her expertise as an addiction specialist applied to him and he had devised his own post-rehab regimen – resuming his work as a police consultant in New York City. With no choice but to accompany her new charge on his jobs, Watson quickly realized she had a knack for playing investigator, and Sherlock found her medical background helpful. Sherlock's police contact, Captain Thomas Gregson, knew from previous experience working with Scotland Yard that Sherlock was brilliant at closing cases and eventually accepted the addition of Watson to his team. Sherlock and Watson work with Detective Marcus Bell, a top-notch investigator with sharp intuition and formidable interrogation skills. With the mischievous Sherlock Holmes running free in New York solving crime, it's simple deduction that he needs someone to keep him grounded, and it's elementary that it's a job for Watson.