In a Mars base, the inhabitants are being infected by a mysterious water creature which takes over its victims. The Doctor is thrust into the middle of this catastrophe, knowing a larger one is waiting around the corner.

When a chance encounter brings soldier John Watson into Sherlock's life, it's apparent the two men couldn't be more different, but Sherlock's intellect coupled with John's pragmatism soon forges an unbreakable alliance as they investigate a series of baffling cases together.