I'll Never Set Foot in That House Again! Volume 3

After solving the Radzuel Empire’s miasma problem, and spending half a year recuperating from a mana fever in the Sargent Margraviate, Chelsea returns to the Royal Research Institute as Glen’s fiancée. Making her social debut at a party attended by the kingdom of Chronowize’s royal family, she makes her very first friend, a noble girl named Noel! During a visit to Noel’s home, they’re attacked by men in black—worshipers of the Proxy, who steal strengthening herbs from the greenhouse that can make monsters go berserk. Their true aim is...the destruction of the spirit trees, and the assassination of the Spirit King Element’s contractor, Chelsea! But once he learns the worshipers’ plans, Glen moves to round up all of the criminals targeting Chelsea before their engagement is officially announced!

[From J-Novel Club]