The Flash -- "The Once and Future Flash" -- FLA319a_0027b.jpg -- Pictured: Ashley Rickards as Rosalind Rosa Dillion -- Photo: Katie Yu/The CW -- © 2017 The CW Network, LLC. All rights reserved.

The Flash -- "The Once and Future Flash" -- FLA319a_0059b.jpg -- Pictured (L-R): Ashley Rickards as Rosalind Rosa Dillion and Grey Damon as Sam Scudder -- Photo: Katie Yu/The CW -- © 2017 The CW Network, LLC. All rights reserved.

The Flash -- "The Once and Future Flash" -- FLA319a_0096b.jpg -- Pictured: Grant Gustin as The Flash -- Photo: Katie Yu/The CW -- © 2017 The CW Network, LLC. All rights reserved.

The Flash -- "The Once and Future Flash" -- FLA319a_0147b.jpg -- Pictured (L-R): Carlos Valdes as Cisco Ramon and Grant Gustin as Barry Allen -- Photo: Katie Yu/The CW -- © 2017 The CW Network, LLC. All rights reserved.

The Flash -- "The Once and Future Flash" -- FLA319a_BTS_0159b.jpg -- Pictured (L-R): Behind the scenes with Tom Cavanagh and Grant Gustin as The Flash -- Photo: Katie Yu/The CW -- © 2017 The CW Network, LLC. All rights reserved.

The Flash -- "The Once and Future Flash" -- FLA319a_BTS_0173b.jpg -- Pictured: Behind the scenes with Tom Cavanagh -- Photo: Katie Yu/The CW -- © 2017 The CW Network, LLC. All rights reserved.

The Flash -- "The Once and Future Flash" -- FLA319a_BTS_0174b.jpg -- Pictured: Behind the scenes with Tom Cavanagh -- Photo: Katie Yu/The CW -- © 2017 The CW Network, LLC. All rights reserved.

The Flash -- "The Once and Future Flash" -- FLA319a_BTS_0181b.jpg -- Pictured: Behind the scenes with Tom Cavanagh and Grant Gustin -- Photo: Katie Yu/The CW -- © 2017 The CW Network, LLC. All rights reserved.

The Flash -- "The Once and Future Flash" -- FLA319a_BTS_0207b.jpg -- Pictured (L-R): Behind the scenes with Grant Gustin and Tom Cavanagh -- Photo: Katie Yu/The CW -- © 2017 The CW Network, LLC. All rights reserved.

The Flash -- "The Once and Future Flash" -- FLA319a_BTS_0227b.jpg -- Pictured (L-R): Behind the scenes with Tom Cavanagh and Grant Gustin -- Photo: Katie Yu/The CW -- © 2017 The CW Network, LLC. All rights reserved.

The Flash -- "The Once and Future Flash" -- FLA319a_BTS_0262b.jpg -- Pictured: Behind the scenes with Grant Gustin -- Photo: Katie Yu/The CW -- © 2017 The CW Network, LLC. All rights reserved.

The Flash -- "The Once and Future Flash" -- FLA319a_BTS_0300b.jpg -- Pictured (L-R): Behind the scenes with Grant Gustin, Tom Cavanagh and Carlos Valdes -- Photo: Katie Yu/The CW -- © 2017 The CW Network, LLC. All rights reserved.

The Flash -- "The Once and Future Flash" -- FLA319b_0007b.jpg -- Pictured: Jesse L. Martin as Detective Joe West -- Photo: Dean Buscher/The CW -- © 2017 The CW Network, LLC. All rights reserved.

The Flash -- "The Once and Future Flash" -- FLA319b_0028b.jpg -- Pictured: Grant Gustin as Barry Allen -- Photo: Dean Buscher/The CW -- © 2017 The CW Network, LLC. All rights reserved.

The Flash -- "The Once and Future Flash" -- FLA319b_0041b.jpg -- Pictured: Jesse L. Martin as Detective Joe West -- Photo: Dean Buscher/The CW -- © 2017 The CW Network, LLC. All rights reserved.