The Devil's Gambit was the 5th and final episode of the 2nd Series of The Mary Morstan Mysteries. It was also the final episode of the entire show. It originally ran in 4 parts between April 21st 2015 - May 7th 2015. The story concludes the series 2 story arc over Professor Moriarty's "Super Simian Serum", as well as setting up the events that eventually led to the main No Place Like Holmes series 1st episode.
The episode opens with Moriarty receiving a letter from a man promising him high riches and rewards if he will give him protection from the persecution he is facing trying to "practise his religion". Moriarty considers the offer but disregards it, feeling he has no need for it.
Meanwhile, Mary and Holmes are trying to uncover the truth behind who is making the strength enhancing serum, and how can they counteract it. Mary eventually comes across a local boxer named Steve Dixie who she discovers is taking the serum. Under the guise of her alter-ego, Horace Moustachio, Mary convinces Dixie to take her under his wing to teach her about boxing and how to get her hands on the serum itself.
Holmes experiments on the recently captured Eduardo Estaban (another serum patient), who has seemingly gone incoherent and raving mad, until he eats a soup made by Mrs. Hudson consisting of turnip, cabbage, cauliflower and mustard greens. However, Estaban soon loses all his senses again once he is given a banana cake by Mrs. Hudson.
As a result of these events, Holmes realises a property in the vegetables (a goitrogen) is blocking the body's thyroid gland from absorbing Iodine. If the body is unable to do this, the potion is no longer able to work, as it requires the thyroid gland to keep pumping hormones to enable it to work.
Holmes and Mary also realise that the banana sets off the serum users, because it is a native food to the langur monkey, and the gland of the monkey reacts violently within the host once introduced to one of its habitual foods.
With this knowledge in mind, Holmes manages to create a cure for the serum which Mary uses against Dixie to remove all trace of his super powers.
With no strength or speed left, Dixie pulls out of the fight that Professor Moriarty has 50,000 pounds riding on, meaning Moriarty is facing complete financial ruin unless he is able to find the money to pay back his backers soon.
He recalls the letter from earlier on in the episode and with no other options available to him, agrees to enter into an alliance with man, who we then learn is Sir Hugo Baskerville.
The exact details of the alliance are still as of yet unclear, but what does become apparent, is that Baskerville's encounter with Holmes 5 years later, might not have been so random after all, and had been planned all along. But for what purpose? That remains to be seen.