The episode will guest-star Paul Blackthorne and Katie Cassidy from Arrow in their roles as Captain Quentin Lance and his daughter Laurel.
You can check out the official synopsis below.
JOE AND CISCO TRAVEL TO STARLING CITY FOR ANSWERS ABOUT DR. WELLS — Joe (Jesse L. Martin) and Cisco (Carlos Valdes) head to Starling City to continue their investigation of Dr. Wells (Tom Cavanagh). While in town, the duo enlists the help of Captain Lance (guest star Paul Blackthorne), and Cisco meets the Black Canary (guest star Katie Cassidy), who asks him for a favor. Meanwhile, back in Central City, Barry (Grant Gustin) races to catch a meta-human named Hannibal Bates (guest star Martin Novotny) who can transform himself into every person he touches – which includes Eddie (Rick Cosnett), Iris (Candice Patton), Caitlin (Danielle Panabaker) and even The Flash. Wendey Stanzler directed the episode written by Ray Utarnachitt & Cortney Norris (#119). Original airdate 4/21/2015.