(2 of 12)

The Flash -- "Monster" -- Image FLA305a_0092b.jpg -- Pictured (L-R): Grant Gustin as The Flash and Candice Patton as Iris West -- Photo: Katie Yu/The CW -- © 2016 The CW Network, LLC. All rights reserved.

(3 of 12)

The Flash -- "Monster" -- Image FLA305a_0107b.jpg -- Pictured: Grant Gustin as The Flash -- Photo: Katie Yu/The CW -- © 2016 The CW Network, LLC. All rights reserved.

(4 of 12)

The Flash -- "Monster" -- Image FLA305a_0120b.jpg -- Pictured: Grant Gustin as The Flash -- Photo: Katie Yu/The CW -- © 2016 The CW Network, LLC. All rights reserved.

(5 of 12)

The Flash -- "Monster" -- Image FLA305a_0151b.jpg -- Pictured: Grant Gustin as The Flash -- Photo: Katie Yu/The CW -- © 2016 The CW Network, LLC. All rights reserved.

(6 of 12)

The Flash -- "Monster" -- Image FLA305b_0019b.jpg -- Pictured: Danielle Panabaker as Caitlin Snow -- Photo: Katie Yu/The CW -- © 2016 The CW Network, LLC. All rights reserved.

(7 of 12)

The Flash -- "Monster" -- Image FLA305b_0040b.jpg -- Pictured: Susan Walters as Dr. Tannhauser -- Photo: Katie Yu/The CW -- © 2016 The CW Network, LLC. All rights reserved.

(8 of 12)

The Flash -- "Monster" -- Image FLA305b_0131b2.jpg -- Pictured: Susan Walters as Dr. Tannhauser -- Photo: Katie Yu/The CW -- © 2016 The CW Network, LLC. All rights reserved.

(9 of 12)

The Flash -- "Monster" -- Image FLA305b_0180b.jpg -- Pictured (L-R): Susan Walters as Dr. Tannhauser and Danielle Panabaker as Caitlin Snow -- Photo: Katie Yu/The CW -- © 2016 The CW Network, LLC. All rights reserved.

(10 of 12)

The Flash -- "Monster" -- Image FLA305b_0245b.jpg -- Pictured: Danielle Panabaker as Caitlin Snow -- Photo: Katie Yu/The CW -- © 2016 The CW Network, LLC. All rights reserved.

(11 of 12)

The Flash -- "Monster" -- Image FLA305b_0286b.jpg -- Pictured: Danielle Panabaker as Caitlin Snow -- Photo: Katie Yu/The CW -- © 2016 The CW Network, LLC. All rights reserved.

(12 of 12)

The Flash -- "Monster" -- Image FLA305b_0370b.jpg -- Pictured (L-R): Danielle Panabaker as Caitlin Snow and Thomas Cadrot as Nigel -- Photo: Katie Yu/The CW -- © 2016 The CW Network, LLC. All rights reserved.