Following the defeat of Adam, Scoobies, Buffy and Spike are looking round the Initiative, Buffy and Spike go into a cubicle and Xander accidentally pushes buttons causing them to fall through the floor into another dimension. After getting together there, they make it back and both Angel and Riley are there with the Scoobies.
It's not Dreanweaver's 'A Different Light' or 'Running Wild' — I just can't find it and it's driving me crazy!.

Hi Lou,

It probably is 'To Save a Lady' by Slaymesoftly here on TSR, on EF, on AO3, on fanfiction.net and The Bloodshedverse.

Hope this it.

Season 5, Time Travel, Claim
Summary:  Ok, here's the sitch.  It's early in a Season V that has been tweaked to suit my purposes.  Spike hasn't had his little revelation about his feelings for Buffy, and she, of course, is clueless.  Riley has had his surgery and begun his macho identity crisis.  Dawn won't be mentioned much because she isn't a big part of this story. Riley has gone back to Iowa for an extended leave, partly to recover from his surgery and partly to figure out what he wants to do next.  He was apprehensive about being gone from Buffy for so long, and not at all reassured when she tells him she will take Spike with her for extra muscle if he's so worried about her.  He reminds her of her words to him before he lost the enhancements that were killing him, when she told him that if she «wanted somebody with super powers, I'd be dating Spike!» She rolls her eyes at his implication and they do not part on the best of terms.

That's the one! Thank you so much, you're a lifesaver!