"Angel" is a spin-off of Gellar's WB vampire "Buffy the Vampire Slayer," which ended its seven-year run last spring.
The WB announced in February "Angel" would not be back next fall, leaving legions of fans to wonder if Gellar would appear on "Angel" to tie up loose ends -- particularly whether her small-screen alter-ego would end up with brooding do-gooder Angel, played by David Boreanaz or witty bad boy Spike, played by James Marsters.
Though Gellar will not make an appearance on the show, the May 5 episode is about Angel and Spike heading to Rome to rescue Buffy from an old foe, The Immortal, at the behest of "Buffy" alum Andrew, played by Tom Lenk, said Zap2it.com.
"Buffy" and "Angel" characters Darla, played by Julie Benz, and Drusilla, played by Juliet Landau, appear in flashback scenes.
The series is set to end May 19.