Why does the efficient set with the Markowitz model extended to include riskfree borrowing and lending have

Question:

Why does the efficient set with the Markowitz model extended to include riskfree borrowing and lending have only one point in common with the efficient set of the Markowitz model without riskfree borrowing and lending? Why are the other points on the "old" efficient set no longer desirable? Explain with words and graphs.