Current:

Barry feels guilty for what happened to Caitlin and Mark comes up with a plan which makes everyone skeptical. Meanwhile, Red Death looms in Central City and commands that failure is not an option. Joe asks Cecile to make a choice which leads Cecile to think about the greater good. Allegra and Chester take a step in a new direction. Lastly, old friends pay an unexpected visit to S.T.A.R. Labs.

Corrected:

Barry feels guilty for what happened to Caitlin, and Mark comes up with a plan that makes everyone skeptical. Meanwhile, Red Death looms in Central City and commands that failure is not an option. Joe asks Cecile to make a choice that leads Cecile to think about the greater good. Allegra and Chester take a step in a new direction. Lastly, old friends pay an unexpected visit to S.T.A.R. Labs.

Summary of edits:

• added comma after "Caitlin" (when "and" conjoins two complete sentences, it must be preceded by a comma)
• changed two instances of "which" to "that" (reference)

I could also nitpick that neither Mark nor his plan can make anyone skeptical, but that's a psychological issue, not a grammatical one. (I would have written it as "Mark comes up with a plan that's met with skepticism." or something like that.)