Months after Sherlock's return where he's taking cases again and John's found a balance between married life and Sherlock, a case goes wrong and Sherlock ends up going for a swim in a very cold river (preferably not the Thames). It triggers flashbacks to him being tortured.

John's got to deal with keeping Sherlock safe from the people they were chasing, as well as he can from the hypothermia that's setting in, calm during the flashback, And getting help from emergency services in a place not easily located.