Sherlock is kidnapped. Over the next few days, his abductors send John photos of the detective bound, beaten up, in varying degrees of distress, and occasionally being sexually abused.
When Sherlock is found, John has to not only help bring Sherlock back to health and get over the traumatic experience, he also has to deal with the fact that all those photos--Sherlock tightly bound, his mouth stretched around a ball gag, foreign fingers reaching toward his trousers--really turned him on.
tl;dr: While Sherlock is kidnapped, John finds photographs of him in distress arousing.
[trigger warning for non-con elements/general nastiness]
When Sherlock is found, John has to not only help bring Sherlock back to health and get over the traumatic experience, he also has to deal with the fact that all those photos--Sherlock tightly bound, his mouth stretched around a ball gag, foreign fingers reaching toward his trousers--really turned him on.
tl;dr: While Sherlock is kidnapped, John finds photographs of him in distress arousing.