Name is authors choice.

Season 3 just after Lovers Walk. Mayors men capture Spike and through the use of drugs and magic put him in a feral state. Demon is in full charge.

Buffy is injured and captured by the mayor's goons and locked in (combination of regular locks and magic) with Spike with the expectation that Spike will kill, possibly after torture, the Slayer. 

Instead Spike (who, demon included, is already in love with Buffy per canon) protects and tends to Buffy. He kills the guards but they still cannot get out of the place they are imprisoned. Because the guards had been human there is food in the place so Spike can make sure Buffy is fed and her wounds tended.

Buffy figures out that Spike for some reason isn't planning to kill or drain her (or even feed). 

Buffy and the non-verbal Spike develop non-hostile relationship and trust as Spike helps Buffy get strong enough to help him find a way to escape.

Must have:

Angel eventually finds out what has happened and (like in S1) decides that Spike would have already killed Buffy or worse. He expects Spike to have a demon like his and also less control. He pretty much just gives up like he did with her battle with the Master and goes home to brood not helping to try to find her.

Scoobies and Giles keep looking and Joyce holds out belief that the "sad young man" wouldn't hurt Buffy that way and that Buffy will be found to be fine.

As Buffy's physical injuries heal the drugs in Spike start to wear off and he has flashes of his normal self enough to plan escape with Buffy. 

They find a way past the magic holding them captive and escape but before they do they each realize they love the other (even if they don't say it).

Spike does something self-sacrificial to enable Buffy to get away but she refuses to leave him behind (while noting that he was willing to sacrifice himself for her).

Buffy preventing anyone from harming Spike when they are free and she clearly sees the differences between Angel and Spike deciding it is much more than the issue of the soul.

Buffy realizing that the demon in Spike clearly loves her too even if she doesn't understand how that can be.

Can have

Joyce laying into Angel over his unwillingness to help. Makes unflattering comparisons between Angel and Spike in front of the gang. She points out how she never felt in danger from Spike but never liked Angel even with his soul.

Scoobies and Giles being disappointed in Angel and someone reminding Giles how Angel did nothing to help Buffy when she went to face the Master.

Faith being the one to tip off Angel about what happened to Buffy.

Scoobies being the ones to eliminate the magic part of the jailing but if you want some other way that's okay too.

Angel accidentally admitting that the soul is not the source of love and that Spike (and many demons) can love. 

Totally must have

Spuffy at least by the end including both admitting they do love each other and plan to make it work