This volume continues with the story of Karma and Nagisa's trip to the space to steal the research data.
Their return brings joy to their classmates. They analyze the research data and realized that the solution is a familiar drug developed by one of their classmates before. At the current stage, this leaves to them to decide whether they want to proceed with the assassination.
The entrance exam is coming soon. Everyone seems decided what they wanted to do after high school except Nagisa. In the meantime, Korosensei's past is catching up and his former student and rival are planning a revenge against them.
The remaining volume continues with the relationships among the students which are beyond friends and classmates. This seems probably occurs to teachers as well.
More reviews can be found on Goodreads: Assassination Classroom, Vol. 18: Valentine's Time (Assassination Classroom #18).