on the Q.T.

met secretly to discuss the invasion plans

侵攻計画を話し合うために秘密裏に会った

the children secretly went to the movies when they were supposed to be at the library

子どもたちは図書館にいるはずのときに、こっそり映画を見に行った

they arranged to meet in secret

彼らは秘密裏に会う手はずを整えた