As Sisera escaped, the narrative focused on an alliance that had been made between the king of Canaan and the family of Heber. This was convenient for Sisera because it gave him an out--he could run for protection and run he did (he "fled on foot"; 4:15, 17). He made his way directly to Heber's house where he was met by Jael, Heber's wife. He demanded aid from Jael and appeared to receive it, although not in the form he demanded: when he asked for a drink of water to quench his thirst he was given milk, which made him sleepy. But before he fell asleep he made another demand of Jael: she was to keep watch for him and if anyone came by asking if a man was inside her tent, she was to tell them no.
Wait a minute--no man inside the tent! What did Sisera consider himself to be? While it's obvious he told Jael to say this for his protection, it is humorous (and a bit ironic) that he undermined his own manhood and "invulnerability" by hiding under a rug and demanding Jael lie for him.
But the final irony came while he was asleep. Jael grabbed a tent peg and drove it through his skull, killing him. The she went out to meet Barak to turn over the dead Sisera.
This is intended to be both ironic and something that showed the glory of God--that God ensured his will was done despite the human alliances made by his enemies.
The hero of the story is Jael--an unlikely hero. This unlikely hero--a woman, a non-Israelite--did the work of God (4:23)! The true hero should have been Barak, or perhaps even Deborah, but it was a non-Israelite who served God in the unlikeliest of ways. This teaches us that no matter how small or insignificant we think we are, God can do great things through us if we are willing to follow him as he leads us. In fact, there is no "small" or "insignificant" with God--he is ready to use anyone who loves him, anyone who is ready to do his work. All you need to do is make yourself available to be led by God.
Will you be a hero for God?