In fact, Milton opens the play with Samson in shackles. Miltons description is told more from Samsons point-of-view than the Biblical story. In the Bible, the description of the imprisonment is objective or told more from the Philistine perspective: “Binding him with bronze shackles, they set him to grinding grain in the prison,” (Judges 16:21).
On the other hand, Milton makes it clear that Samsons imprisonment is one of psychological as well as physical torture: “The Dungeon of thy self; thy Soul / (Which Men enjoying sight oft without cause complain) / Imprisond now indeed, / in real darkness of the body dwells, / Shut up from outward light,” (lines 156-160).
Delilah features more prominently in Miltons story than in the Bible. Milton offers some closure for the audience familiar with the Biblical tale, which does not allow Samson to express his anger.
In the Biblical account, the reader infers Delilahs betrayal; in Miltons story, her unethical actions are spelled out as the root cause for Samsons suffering. First, Samson cries, “I yielded, and unlockd her all my heart,” (line 407).
Later, Samsons anger is palpable when he tells Delilah he has a “sudden rage to tear thee joint by joint,” (953).
Samson still emerges as the just, godlike hero that he does in the Bible. He delivers a noble statement to Delilah: “At distance I forgive thee, go with that,” (line 954).
The Samson and Delilah story is enhanced in Miltons play, mainly by allowing Samson to express his anger at a woman who betrayed him.
Bible, New International Version.
Milton. Samson Agonistes. Retrieved online: http://www.dartmouth.edu/~milton/reading_room/samson/drama/index.shtml
Milton, Samson Agonistes, lines 125-131
Milton, lines 28-29