Snape has been sentenced to ten years hard labour for his actions in the war, but either Hogwarts itself has other ideas, or someone is trying to cause trouble. An elf has died and other lives are threatened. Snape has to solve the mystery to regain his freedom (or as much freedom as he is likely to get.)
The gentle, unspoken way the relationship between Minerva and Severus develops is delicious, and their tentativeness fits the situation so perfectly.
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