Gunther's nasty knights, led by Hagen, develop an evil plan. They trick Siegfried into thinking Gunther's kingdom is coming under attack once again, and once again Siegfried offers to play hero. Kriemhild happily (stupidly) watches him go, proud that her husband can help her brother.
Before they go, Hagen pauses to flirt a bit with Kriemhild, promising to protect Siegfried during battle (and by protect he means stab him in the back). Kriemhild puts her foot in her mouth once again (maybe this is where the women should be seen and not heard rule comes from), saying that because he once bathed in the blood of a dragon, Siegfried can only be wounded by the spear, not by any weapon that cuts, and only on one spot between his shoulders.
Hagen tells Kriemhild to sew a token there in that spot, so he "knows where Siegfried needs to be protected" *shifty eyes*.
Kriemhild, in an effort to be helpful, sews a big freaking bulls-eye onto Siegfried's jacket, so that his weak spot can be apparent to the whole world. (SHE'S SO SMART!).
Right after they leave, a messenger rides up saying : "Good news, chaps! No longer under attack!" (not exactly sure what the point in that ruse was.) So, Gunther, under Hagens guidance, decides to go hunting instead, and Siegfried agrees to go with.