The point of ”Oh, what a tangled web we weave…” is that it is better to tell the truth in from the start than be involved in deception.
Marmion is a story about an evil man named Lord Marmion who desires a woman already engaged to another man, Sir Ralph De Wilton. Lord Marmion is helped by his mistress, Constance, to forge documents implicating De Wilton with treason.
Constance assisted Marmion with his fraud under the mistaken notion that he would appreciate and love her.
To make a long story short, Marmion abandons Constance, so she seeks her revenge. She went to the proper authority to provide documents proving that De Winton is not a traitor. In end De Winton is restored his previous place of honor as a knight.
Before De Winton has the opportunity to get revenge on Marmion for his mendacious behavior, Marmion dies in a battle and De Winton in the end marries his fiancee.
Moral of “Oh, what a tangled web we weave“? Be honest. You won’t have people turn against you and you don’t have to remember what is the truth and what is a lie.