By chance, Rikuto Sekiya meets a women with the power to send people into the past. However, it seems there are limitations: she can only send them back for a period of 20 minutes.
While wondering "Can the past really be changed in only 20 minutes?" Rikuto is sent into a Time Slip by the woman.

Rikuto wishes to return to the time he spent with two girls from his past. A tragic incident occurred then. He has traveled to the past with the hopes of averting this tragedy.

However, the true character of a man determined to change the past is...?

The two girls from his past, and the mysterious woman who sent him there... Rikuto will trifle with the very fabric of their beings.

From Chibicon.net