Spin The Bottle
In an attempt to return Cordelia's memory, Lorne arrives with a magic bottle and a memory spell. The whole gang, plus Wesley, sit down together and perform the spell, but it backfires when Cordelia steps on the bottle.
Suddenly, eveyone's memory reverts to when they were seventeen, with the exception of Lorne, who instead has passed out behind the counter.
The gang bicker with one another, discovering Lorne in the process. Wesley convinces them there's a vampire in the hotel somewhere, which they must kill as a test. They split up to find it.
Angel, or Liam as he believes himself to be, realises that he's a vampire when he comes face to non-reflection in a mirror. He attempts to hide this from the group, hitting Lorne to keep him quiet. As he chases Cordelia, Connor appears and intervenes, and the two fight it out in the hotel kitchens. Only the intervention of Fred, her memory restored, stops them.