Instead of staying around London, Mary assumes the worst when John refuses to talk to her, and as soon as she's sure Sherlock will pull through okay, she leaves a note and vanishes.

With a ticking clock on the pregnancy, John and Sherlock attempt to track her down while simultaneously taking out Mangussen so they can bring her home and she can have the baby in peace/safety.

(John/Mary or Sherlockary threesome plz.)