Sherlock comes across a alpha/alpha relationship during a case and it triggers something within Sherlock. It's not exactly unheard of, but seeing it actually in real life for Sherlock makes his imagination run wild. Sherlock develops this obsession of experiencing another alphas knot within him. He starts masturbating frequently, plays with his own knot and much to John's horror, Sherlock starts actively trying to flirt with any alpha he sees in hopes that one of them will bite and fuck Sherlock.

Of course, as a doctor, John knows all the horrible ways it could turn out and hurt his best friend and he's the one who's going to be there when Sherlock is hurt. So he begs Sherlock to stop this nonsense. Sherlock is stubborn until he thinks john could do it! With his medical knowledge, John could prepare and knot him!

And so it eventually happens and it's terrible. It hurts so much and Sherlock is far from aroused, his erection lost long before full penetration... But he's determined and the actual knotting sends Sherlock into tears. But after its all over... After a couple of weeks... Sherlock wants to try again. And by this time they've both admitted to being attracted to each other.