After light has given up his Death Note and both he and Misa have no memories of their usage of them, killings begin once again.  Forced to admit he's at a loss L releases them but on one condition.  Mr Yagami drives Light and Misa to a location and tells them that he is going to kill them and then himself.  They act in such an innocent way that, when the whole thing is revealed to be another test by L, they appear to truly be innocent.  L revises his idea that they were just used by the true Kira but were in fact Kira's themselves at some point.