“Flash vs. Arrow” — (8:00-9:00 p.m. ET) (TV-PG, V) (HDTV) PART ONE OF THE EPIC CROSSOVER EVENT WITH THE FLASH AND ARROW — Barry (Grant Gustin) is thrilled when Oliver (guest star Stephen Amell), Felicity (guest star Emily Bett Rickards) and Diggle (guest star David Ramsey) come to Central City to investigate a case involving a deadly boomerang. Excited about teaming up with his friend, Barry asks Oliver if he’d like to help him stop Ray Bivolo (guest star Patrick Sabongui), the meta-human Barry is currently tracking. Bivolo causes people to lose control of their emotions and has been using that skill to rob banks. Unfortunately, the superhero partnership doesn’t go as smoothly as Barry expected. When Oliver tells Barry he still has a lot to learn, Barry sets out to prove him wrong by attempting to stop Bivolo alone. However, when Bivolo infects Barry and sets him on a rage rampage, everyone is in danger, and the only one who can stop him is the Arrow. Meanwhile, Iris (Candice Patton) is furious when Eddie (Rick Cosnett) tries to get a task force to stop The Flash, Joe and Dr. Wells (Tom Cavanagh) agree the Arrow is a bad influence on Barry, and Caitlin and Cisco deal with a new team in S.T.A.R. Labs. Glen Winter directed the episode with story by Greg Berlanti & Andrew Kreisberg and teleplay by Ben Sokolowski & Brooke Eikmeier

“The Man in the Yellow Suit” — (8:00-9:01:30 p.m. ET) (TV-14, V) (HDTV) BARRY COMES FACE-TO-FACE WITH THE MAN IN THE YELLOW SUIT/AMANDA PAYS RETURNS TO THE FLASH — Barry (Grant Gustin) finds himself face-to-face with his nemesis, the man in the yellow suit, a.k.a. Reverse Flash, who killed his mother. Barry is frustrated when the Reverse Flash escapes, but Dr. Wells (Tom Cavanagh) and Cisco (Carlos Valdes) come up with a plan to trap him. All they need is bait, so they turn to Dr. Tina McGee (guest star Amanda Pays) from Mercury Labs to help. Meanwhile, it’s Christmas at The West household and Iris (Candice Patton) is in full holiday cheer. Eddie (Rick Cosnett) gives her a surprising gift but also leaves her with a question that could change her life. Joe (Jesse L. Martin) joins the search for the man in yellow and ends up in the crossfire. Caitlin (Danielle Panabaker) finds Ronnie (Robbie Amell). Ralph Hemecker directed the episode written by Todd Helbing & Aaron Helbing

After the disastrous first bout, Barry will set his sights on testing the limitations of his powers in a bid to match his nemesis. “We’re exploring the vibrating a lot going forward and what the vibrating can do,” Grant Gustin says. “And how to slow things down, see things and survey situations with time stopped, essentially.” Will those powers also include time travel? “Maybe,” Gustin says with the highest pitch voice of someone trying to keep a secret.