S04E16: 'On The Head Of A Pin'

Castiel and Uriel resurface to seek Dean's help in discovering who is responsible for murdering seven angels from their garrison.

Suspecting the demon Alastair to know something, they ask Dean to revisit the torture skills he learned in Hell in order to extract the information from his old acquaintance. Initially reluctant, he soon warms to the task when Alastair forces him to relive the nightmare of his time down below.

Castiel later confesses that his superiors are beginning to question his relationship with Dean, adding that he is concerned about the nature of the orders that are coming from above.

The situation also attracts the attention of Anna, who tries to stop the torture, but not before Alastair drops a massive bombshell about the true extent of Dean's role in Lillith's plan to bring about the apocalypse.

Elsewhere, the reason behind Sam's dependence on Ruby is revealed and Castiel faces a nasty shock when the real perpetrator of the killings is revealed.