Dexter Season 5 Episode 5 (Dexter s05e05) with title “First Blood” aired on Sunday, October 24th 2010 at 09:00 pm via Showtime Network.
Here is summary for Dexter Season 5 Episode 5 :

When Dexter is saddled with an unwanted conspirator, he is forced to make some hard decisions about her fate; meanwhile, Dexter wonders if Rita’s death has awakened a darkness inside Harrison; due to Quinn’s recent suspension, Debra works the Santa Muerte case alone, leaving Quinn time to enlist an old friend to look into Dexter.