From MangaHelpers:

A highly skilled, nameless assassin has taken on a job to kill the son of a highly regarded professor. He has two months to do the deed, but he is certain that it will take him mere days. However, when the assassin goes to scout out his kill, he is struck violently in the head with a volleyball from his target's club practice. Now the assassin remembers nothing, except that his target, Mizuno Yukiaki, is a person that was important to him for some reason. Yukiaki decides that the best thing to do is to convince this scary-looking, amnesiac stranger that he is actually a member of their club named Sam. Now Sam attempts to 'remember' how to play volleyball with his teammates, not even knowing that the deadline for his kill is creeping nearer! [tethysdust]