Maiden Holmes Episode 18

Su Ci decides to pretend to be the Princess in order to lure the culprit out. At night, an assailant enters her room, but is caught by Pei Zhao. It turns out the assailant is Che Yan, the servant of Shuomu. Che Yan explains that he’s ordered to assassinate the Princess, but he’s not the culprit of the poisoning. Su Ci is poisoned. To cure Su Ci, the group have to find out which four drugs that are used. After some deduction, they realize the culprit is Princess’ teacher. But who’s the mastermind? (Source: Viu)